librbd: fix recursive locking on owner_lock in ImageDispatch

librbd: fix recursive locking on owner_lock in ImageDispatch

needs_exclusive_lock() calls acquire_lock() with owner_lock held.
If lock acquisiton races with lock shut down, ManagedLock completes
ImageDispatch context directly and dispatch is retried immediately on
the same thread (due to DISPATCH_RESULT_RESTART).  This results in
recursion into needs_exclusive_lock() and, barring locking issues, can
lead to unbounded stack growth if lock shut down takes its time.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>