xfs: refactor quota type testing

Certain functions can only act upon one quota type, so refactor those
functions to use switch statements, in keeping with all the other high
level xfs quota api calls.

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