xfs: refactor xfs_parseags()

Refactor xfs_parseags(), move the entire token case block to a separate
function in an attempt to highlight the code that actually changes in
converting to use the new mount api.

Also change the break in the switch to a return in the factored out
xfs_fc_parse_param() function.

Signed-off-by: Ian Kent <raven@themaw.net>
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>