xfs: rename xfs_fc_* back to xfs_fs_*

Get rid of this one-off namespace since we're done converting things to
fscontext now.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>