librbd/cache/pwl/ssd/WriteLog: decrement m_bytes_allocated when retiring

librbd/cache/pwl/ssd/WriteLog: decrement m_bytes_allocated when retiring

Currently if ssd cache is filled to capacity, all future I/O hangs
indefinitely because even though the cache eventually becomes clean
and retires enough entries to get back under RETIRE_HIGH_WATER, this
isn't communicated to AbstractWriteLog::check_allocation().

trivial fix: indentation https://www.diffchecker.com/9Vg9hgdl
Fixes: https://tracker.ceph.com/issues/50560
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit e2bbf4167fc8816d329ceae5c07c9e61599d9a17)