dlm: improve rsb searches

By pre-allocating rsb structs before searching the hash
table, they can be inserted immediately.  This avoids
always having to repeat the search when adding the struct
to hash list.

This also adds space to the rsb struct for a max resource
name, so an rsb allocation can be used by any request.
The constant size also allows us to finally use a slab
for the rsb structs.

Signed-off-by: David Teigland <teigland@redhat.com>