NFS: switch the callback service back to non-pooled.

Now that thread management is consistent there is no need for
nfs-callback to use svc_create_pooled() as introduced in Commit
df807fff

 ("NFSv4.x/callback: Create the callback service through
svc_create_pooled").  So switch back to svc_create().

If service pools were configured, but the number of threads were left at
'1', nfs callback may not work reliably when svc_create_pooled() is used.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>