scripts/pahole-flags.sh change return to exit 0

BugLink: https://bugs.launchpad.net/bugs/2035123

[Impact]

When building the Jammy linux-bluefield kernel tree on a system without pahole installed, the following warning is emitted:

./scripts/pahole-flags.sh: line 7: return: can only `return' from a function or sourced script

scripts/pahole-flags.sh attempts to return from an if statement that is not within a function, and generates a warning.

The fix is straightforward, changing return to an exit 0.

--- a/scripts/pahole-flags.sh
+++ b/scripts/pahole-flags.sh
@@ -4,7 +4,7 @@
 extra_paholeopt=

 if ! [ -x "$(command -v ${PAHOLE})" ]; then
- return
+ exit 0
 fi

[Testcase]

Clone the linux-bluefield kernel tree and build it on a arm64 system without
pahole installed.

A test kernel is available with the fix applied in:

https://launchpad.net/~mruffell/+archive/ubuntu/sf368560-test

Both linux-bluefield and ubuntu-jammy build correctly.

[Fix]

This was fixed by Linus Torvalds in the following merge commit:

commit fc02cb2b37fe2cbf1d3334b9f0f0eab9431766c4
Merge: bfc484fe6abb 84882cf72cd7
Author: Linus Torvalds <email address hidden>
Date: Tue Nov 2 06:20:58 2021 -0700
Subject: Merge tag 'net-next-for-5.16' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc02cb2b37fe2cbf1d3334b9f0f0eab9431766c4

Note, the original commit does not have the fix included:

commit 9741e07ece7c247dd65e1aa01e16b683f01c05a8
Author: Jiri Olsa <email address hidden>
Date: Fri Oct 29 14:57:29 2021 +0200
Subject: kbuild: Unify options for BTF generation for vmlinux and modules
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9741e07ece7c247dd65e1aa01e16b683f01c05a8

The Ubuntu kernel cherry-picked the original commit and did not pick up the
silent fix made in the merge commit. Submitting the silent fix as a SAUCE patch
with changelog describing the change.

Note: I know SAUCE patches are bad, but in this scenario, to revert the initial commit and re-apply the fixed version would require us to revert two additional dependency commits, making six patches to review, vs, a one line change in a SAUCE commit that has a real changelog entry.

[Where problems could occur]

The Ubuntu kernel is built with pahole enabled, and requires pahole to be installed as a build dependency. It is extremely unlikely that any users are disabling pahole at build time, apart from linux-bluefield engineers.

If a regression were to occur, engineers would see errors during build time about scripts/pahole-flags.sh not executing properly.

[Other info]

Linus remarked about the issue in the following lkml discussion:

https://lore.kernel<email address hidden>/
https://lore.kernel<email address hidden>/

This was silently Incorporated into the linux-stable commit:

commit 0baced0e0938f2895ceba54038eaf15ed91032e7 5.15.y
From: Jiri Olsa <email address hidden>
Date: Sun, 4 Sep 2022 15:19:00 +0200
Subject: kbuild: Unify options for BTF generation for vmlinux and modules
Link: https://github.com/gregkh/linux/commit/0baced0e0938f2895ceba54038eaf15ed91032e7