xfs: make sure syncfs(2) passes back super_operations.sync_fs errors

xfs: make sure syncfs(2) passes back super_operations.sync_fs errors

This is a regression test to make sure that nonzero error returns from
a filesystem's ->sync_fs implementation are actually passed back to
userspace when the call stack involves syncfs(2).

[zlang@ add '_supported_fs xfs' in case]

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>