xfs: factor out the common lock flags assert

There are similar lock flags assert in xfs_ilock(), xfs_ilock_nowait(),
xfs_iunlock(), thus we can factor it out into a helper that is clear.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>