librados: make querying pools for selfmanaged snaps reliable

librados: make querying pools for selfmanaged snaps reliable

If get_pool_is_selfmanaged_snaps_mode() is invoked on a fresh RADOS
client instance that still lacks an osdmap, it returns false, same as
for "this pool is not in selfmanaged snaps mode".  The same happens if
the pool in question doesn't exist since the signature doesn't allow to
return an error.

The motivation for this API was to prevent users from running "rados
cppool" on a pool with unmanaged snapshots and deleting the original
thinking that they have a full copy.  Unfortunately, it's exactly
"rados cppool" that fell into this trap, so no warning is printed and
--yes-i-really-mean-it flag isn't enforced.

Fixes: https://tracker.ceph.com/issues/63607
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>