dm bio prison: switch to using a red black tree

Previously it was using a fixed sized hash table.  There are times
when very many concurrent cells are held (such as when processing a very
large discard).  When this happens the hash table performance becomes
very poor.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc drivers/md/dm-bio-prison.c
Simple merge

diff --cc drivers/md/dm-bio-prison.h
Simple merge

diff --cc drivers/md/dm-cache-target.c
Simple merge

diff --cc drivers/md/dm-thin.c
Simple merge