generic/371: disable speculative preallocation regressions on XFS

generic/371: disable speculative preallocation regressions on XFS

Once in a very long while, the fallocate calls in this test will fail
due to ENOSPC conditions.  While in theory this test is careful only to
allocate at most 160M of space from a 256M filesystem, there's a twist
on XFS: speculative preallocation.

The first loop in this test is an 80M appending write done in units of
4k.  Once the file size hits 64k, XFS will begin speculatively
preallocating blocks past the end of the file; as the file grows larger,
so will the speculative preallocation.

Since the pwrite/rm loop races with the fallocate/rm loop, it's possible
that the fallocate loop will free that file just before the buffered
write extends the speculative preallocation out to 160MB.  With fs and
log overhead, that doesn't leave enough free space to start the 80MB
fallocate request, which tries to avoid disappointing the caller by
freeing all speculative preallocations.  That fails if the pwriter
thread owns the IOLOCK on $testfile1, so fallocate returns ENOSPC and
the test fails.

The simple solution here is to disable speculative preallocation by
setting an extent size hint if the fs is XFS.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>