xfs: Change xfs_dquot_acct to be a 2-dimensional array

In preparation for combined pquota/gquota support, for the sake
of readability, change xfs_dquot_acct to be a 2-dimensional array.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>