xfs: clear log incompat feature bits when the log is idlelog-use-incompat-features_2021-08-25

When there are no ongoing transactions and the log contents have been
checkpointed back into the filesystem, the log performs 'covering',
which is to say that it log a dummy transaction to record the fact that
the tail has caught up with the head.  This is a good time to clear log
incompat feature flags, because they are flags that are temporarily set
to limit the range of kernels that can replay a dirty log.

Since it's possible that some other higher level thread is about to
start logging items protected by a log incompat flag, we create a rwsem
so that upper level threads can coordinate this with the log.  It would
probably be more performant to use a percpu rwsem, but the ability to
/try/ taking the write lock during covering is critical, and percpu
rwsems do not provide that.

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

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