dm ioctl: replace device hash with red-black tree

For high numbers of DM devices the 64-entry hash table has non-trivial
overhead. Fix this by replacing the hash table with a red-black tree.
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>