UPSTREAM: dm bufio: switch from a huge hash table to an rbtree

Converting over to using an rbtree eliminates a fixed 8MB allocation
from vmalloc space for the hash table.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 4e420c452b11edf9d510c8180ac66f529e5b6206)
(this commit from v3.19 is needed to avoid wasting a huge amount of
 memory when using dm-verity, especially on both the system and vendor
 partitions with forward error correction enabled)
Change-Id: Ia48049bb4ed1b766e108bb73619f3939b6f0da90
Signed-off-by: Eric Biggers <ebiggers@google.com>