FATAL error fsyncing in fileio with mode=mmap

When using fileio with file-io-mode=mmap, sysbench fails to fsync the files with the rather cryptic message:

FATAL: Failed to fsync file! id: 0 fd: 3 errno = 0 (Success)

Notice errno is not activated, because fsync() actually succeded. The error comes from a skewered sysbench/tests/fileio/sb_fileio.c's file_sync() return value for mode=mmap. The fix is rather trivial, needs a return sentence for the case different that the catch-all return 1 at the end of the function.

To reproduce:

$ sysbench --test=fileio --file-num=1 --file-total-size=1M prepare
$ sysbench --test=fileio --file-num=1 --file-total-size=1M --file-io-mode=mmap --file-test-mode=seqwr run