krbd: rewrite "already mapped" code

krbd: rewrite "already mapped" code

The "already mapped" code, introduced in commit d6a66fc8f49b ("rbd:
before rbd map, warn if the image is already mapped") is broken:
because of a use-after-free on attribute strings, the warning isn't
even printed half the time.

Rewrite making use of udev enumeration filters and fix the interface
while at it.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>