xfs: online repair for fs summary counters

A longstanding deficiency in the online fs summary counter scrubbing
code is that it hasn't any means to quiesce the incore percpu counters
while it's running.  There is no way to coordinate with other threads
are reserving or freeing free space simultaneously, which leads to false
error reports.  Right now, if the discrepancy is large, we just sort of
shrug and bail out with an incomplete flag, but this is lame.

For repair activity, we actually /do/ need to stabilize the counters to
get an accurate reading and install it in the percpu counter.  To
improve the former and enable the latter, allow the fscounters online
fsck code to perform an exclusive mini-freeze on the filesystem.  The
exclusivity prevents userspace from thawing while we're running, and the
mini-freeze means that we don't wait for the log to quiesce, which will
make both speedier.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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