Use of empty path_cond in AppArmor kernel code can lead to unexpected permission check results

IMPORTANT: Bug #1620635 should be fixed at the same time as this bug. Fixing
this bug will likely result in broken users of libapaprmor's
aa_query_file_path() so those applications will need an available alternative.

The AppArmor kernel code defines the path_cond struct as:

 struct path_cond {
  kuid_t uid;
  umode_t mode;
 };

There are several instances where a path_cond instance is initialized as:

 struct path_cond cond = { };

This results in a path_cond.uid of 0. The cond variable is then sometimes
passed to functions which conditionalize on the value of path_cond.uid.

An example of this can be seen in callers of aa_compute_fperms(). If the passed
in path_cond is simply zero-initialized and the fsuid of the current process is
0 (root), then the permissions are mapped differently than if the fsuid of the
current process was non-zero (non-root).

That's a problem in profile_query_cb() because it results in any "owner"
conditionals being ignored when a root-owned process performs a file
permissions query via libapparmor's aa_query_file_path().

Instead of initializing a path_cond struct to all zeroes, path_cond.uid should
be initialized to INVALID_UID so that it is impossible for the current task's
fsuid to match path_cond.uid.

Here are all of the instances I could find where a path_cond struct is
initialized to all zeroes:

$ git grep -p "path_cond.*{ }" security/apparmor
security/apparmor/af_unix.c=static inline int unix_fs_perm(const char *op, u32 mask, struct aa_label *label,
security/apparmor/af_unix.c: struct path_cond cond = { };
security/apparmor/apparmorfs.c=static void profile_query_cb(struct aa_profile *profile, struct aa_perms *perms,
security/apparmor/apparmorfs.c: struct path_cond cond = { };
security/apparmor/domain.c=static int label_compound_match(struct aa_profile *profile,
security/apparmor/domain.c: struct path_cond cond = { };
security/apparmor/domain.c=static int label_components_match(struct aa_profile *profile,
security/apparmor/domain.c: struct path_cond cond = { };
security/apparmor/lsm.c=static int common_perm_rm(const char *op, struct path *dir,
security/apparmor/lsm.c: struct path_cond cond = { };