xfs: refactor default quota limits by resource

Now that we've split up the dquot resource fields into separate structs,
do the same for the default limits to enable further refactoring.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>