bcache: Add a real GC_MARK_RECLAIMABLE

This means the garbage collection code can better check for data and metadata
pointers to the same buckets.

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

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

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

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

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