xfs-4.18: quota refactor

Hi all,

Here's a series to refactor and clean out the incohesive mess that xfs_qm_dqget has become. I'm reposting this per Christoph request since the v2 thread became very hairy and disorganized.

I've distinguished three contexts in which dqget can be called, and converted them into separate externally visible interfaces -- _qm_dqget_next, which returns the first initialized dquot with an id at least as high as the id passed in; _qm_dqget_inode, which returns the dquot for a given (inode, type); and so _qm_dqget now only returns the dquot for a given (id, type). I removed unused parameters from the dqattach functions, renamed _qm_dqiterate (since it now only knows how to zap dquot counters during quotacheck), and added a real function to iterate all dquots of a given type. I also re-hid _qm_dqread in favor of a new _qm_dqget_uncached which has a better description of why we bypass the dquot cache.

This means that all _dqget variants no longer take QMOPT flags, and QMOPT_NEXT is gone. XFS_QMOPT_DQALLOC is now a boolean and can only be specified for functions where it makes sense.

This whole thing will be in for-next tomorrow, with any luck.

--D