bcache: Avoid deadlocking in garbage collection

Not a complete fix - we could still deadlock if btree_insert_node() has
to split...

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

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

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

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