librbd/cache/pwl/ssd: avoid corrupting m_first_free_entry

librbd/cache/pwl/ssd: avoid corrupting m_first_free_entry

In append_op_log_entries(), new_first_free_entry is read after
append_ops() returns.  This can result in accessing freed memory
because all I/Os may complete and append_ctx callback may run
by the time new_first_free_entry is read.  Garbage value gets
written to m_first_free_entry and depending on the circumstances
it may allow AbstractWriteLog code to accept more dirty user data
than we have space for.  Luckily we usually crash before then.

Fixes: https://tracker.ceph.com/issues/50832
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>