librbd: fix pool validation lockup

librbd: fix pool validation lockup

Concurrent rbd_pool_init() or rbd_create() operations on an unvalidated
(uninitialized) pool trigger a lockup in ValidatePoolRequest state
machine caused by blocking selfmanaged_snap_{create,remove}() calls.
There are two reactor threads by default (librados_thread_count) but we
effectively need N + 1 reactor threads for N concurrent pool validation
requests, especially for small N.

Switch to aio_selfmanaged_snap_{create,remove}().  At the time this
code was initially written, these aio variants weren't available.  The
workqueue offload introduced later worked prior to the move to asio in
pacific.

Fixes: https://tracker.ceph.com/issues/52537
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>