bio.h: If they MUST be inlined, then use __always_inline

bvec_kmap_irq() and bvec_kunmap_irq() comments say they MUST be inlined,
so mark them as __always_inline.

Signed-off-by: Alberto Bertogli <albertito@blitiri.com.ar>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>