block: Advertise BDRV_REQ_NO_FALLBACK in filter drivers

Filter drivers that support .bdrv_co_pwrite_zeroes can safely advertise
BDRV_REQ_NO_FALLBACK because they just forward the request flags to
their child node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>