xfs: move xfs_fc_parse_param() above xfs_fc_get_tree()

Grouping the options parsing and mount handling functions above the
struct fs_context_operations but below the struct super_operations
should improve (some) the grouping of the super operations while also
improving the grouping of the options parsing and mount handling code.

Lastly move xfs_fc_parse_param() and related functions down to above
xfs_fc_get_tree() and it's related functions.

But leave the options enum, struct fs_parameter_spec and the struct
fs_parameter_description declarations at the top since that's the
logical place for them.

This is a straight code move, there aren't any functional changes.
Signed-off-by: NIan Kent <raven@themaw.net>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>