librbd: fix ExclusiveLock::accept_request() when !is_state_locked()

librbd: fix ExclusiveLock::accept_request() when !is_state_locked()

To accept an async request, two conditions must be met: a) exclusive
lock must be a firm STATE_LOCKED state and b) async requests shouldn't
be blocked or if they are blocked there should be an exception in place
for a given request_type.  If a) is met but b) isn't, ret_val is set
to m_request_blocked_ret_val, as expected -- the reason for denying
the request is that async requests are blocked.  However, if a) isn't
met, ret_val also gets set to m_request_blocked_ret_val.  This is wrong
because the reason for denying the request in this case isn't that
async requests are blocked (they may or may not be) but a much heavier
circumstance of exclusive lock being in a transient state or not held
at all.

In such scenarios, whether async requests are blocked or not isn't
relevant and ExclusiveLock::accept_request() behaving otherwise can
lead to bogus "duplicate lock owners detected" errors getting raised
during an attempt to handle any maintenance operation notification in
ImageWatcher::handle_operation_request().  This error isn't considered
retryable so the entire operation that needed the exclusive lock would
be spuriously failed with EINVAL.

Fixes: https://tracker.ceph.com/issues/74168
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>