bcache: Advertise that flushes are supported

commit 54d12f2b4fd0f218590d1490b41a18d0e2328a9a upstream.

Whoops - bcache's flush/FUA was mostly correct, but flushes get filtered
out unless we say we support them...

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