xfs: use a per-resource struct for incore dquot data

Introduce a new struct xfs_dquot_res that we'll use to track all the
incore data for a particular resource type (block, inode, rt block).
This will help us (once we've eliminated q_core) to declutter quota
functions that currently open-code field access or pass around fields
around explicitly.

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>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>