xfs: avoid ilock games in the quota scrubber

Refactor the quota scrubber to take the quotaofflock and grab the quota
inode in the setup function so that we can treat quota in the same
"scrub in the context of this inode" (i.e. sc->ip) manner as we treat
any other inode.  We do have to drop the quota inode's ILOCK_EXCL to use
dqiterate, but since dquots have their own individual locks the ILOCK
wasn't helping us anyway.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>