xfs: move xfs_trans_inode.c to libxfs/

Userspace now has an identical xfs_trans_inode.c which it has already
moved to libxfs/ so do the same move for kernelspace.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>