rbd: fix ceph_pg_poolid_by_name()

rbd: fix ceph_pg_poolid_by_name()

Currently ceph_pg_poolid_by_name() returns an int, which is used to
encode a ceph pool id.  This could be a problem because a pool id
(at least in some cases) is a 64-bit value.  We have a defined pool
id value that represents "no pool," and that's a very sensible
return value here.

This patch changes ceph_pg_poolid_by_name() to return a 64-bit
pool id value, or CEPH_NOPOOL if the named pool is not found.

The patch also gratuitously renames the function, separating "pool"
from "id" in the name by an underscore.

Signed-off-by: Alex Elder <elder@inktank.com>