bcache: Remove/fix some header dependencies

In the process of disentagling/libraryizing bset.c from the rest of the
bcache code.

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

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

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

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