bio: define __BIOVEC_PHYS_MERGEABLE

Define __BIOVEC_PHYS_MERGEABLE as the default implementation of
BIOVEC_PHYS_MERGEABLE, so that its available for reuse within an
arch-specific definition of BIOVEC_PHYS_MERGEABLE.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>