bcache: Convert allocator thread to kthread

Using a workqueue when we just want a single thread is a bit silly.

Signed-off-by: Kent Overstreet <koverstreet@google.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/super.c
Simple merge