netfs, cifs: Improve some debugging bits

Improve some debugging bits:

 (1) The netfslib _debug() macro doesn't need a newline in its format
     string.

 (2) Display the request debug ID and subrequest index in messages emitted
     in smb2_adjust_credits() to make it easier to reference in traces.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>