bcache: FUA fixes

commit e49c7c374e7aacd1f04ecbc21d9dbbeeea4a77d6 upstream.

Journal writes need to be marked FUA, not just REQ_FLUSH. And btree node
writes have... weird ordering requirements.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>