bcache: Convert btree_iter to struct btree_keys

More work to disentangle bset.c 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/btree.h
Simple merge

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

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