xfs: make quota reservations for directory changes

A couple of weeks ago, I was triaging an unexpected quota counter
failure report coming from xfs_repair after the online repair code
rebuilt a directory.  It turned out that the XFS implementation of
linkat does not reserve any quota, which means that a directory
expansion can increase the quota block count beyond the hard limit.
Similar problems exist in the unlink, rmdir, and rename code, so we'll
fix those problems at the same time.

Note: this series does not try to fix a similar bug in the swapext code,
because estimating the necessary quota reservation is very very tricky.
I wrote all that estimation code as the first part of the atomic extent
exchange patchset, so I'll leave that there.

v2: fix unlink and rename, since Dave suggested it
v3: enhance comments, clarify variable names
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