bcache: Convert debug code to btree_keys

More work to disentangle various code from struct btree

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc drivers/md/bcache/bset.c
Simple merge

diff --cc drivers/md/bcache/bset.h
Simple merge

diff --cc drivers/md/bcache/btree.c
Simple merge

diff --cc drivers/md/bcache/debug.c
Simple merge

diff --cc drivers/md/bcache/debug.h
Simple merge

diff --cc drivers/md/bcache/extents.c
Simple merge

diff --cc drivers/md/bcache/extents.h
Simple merge

diff --cc drivers/md/bcache/super.c
Simple merge

diff --cc drivers/md/bcache/util.h
Simple merge