os/bluestore/bluefs: Fix improper vselector tracking in _flush_special()

os/bluestore/bluefs: Fix improper vselector tracking in _flush_special()

Moves vselector size tracking outside _flush_special().
Function _compact_log_async...() updated sizes twice.
Problem could not be solved by making second modification of size just update,
as it will possibly disrupt vselector consistency check (_vselector_check()).
Feature to track vselector consistency relies on the fact that either log.lock or nodes.lock
are taken when the check is performed. Which is not true for _compact_log_async...().

Now _flush_special does not update vselector sizes by itself but leaves the update to
the caller.

Fixes: https://tracker.ceph.com/issues/54248
Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>