xfs: drop unnecessary setfilesize helper

xfs_setfilesize() is the only remaining caller of the internal
__xfs_setfilesize() helper. Fold them into a single function.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>