block: Turn bvec_k{un,}map_irq() into static inline functions

Convert bvec_k{un,}map_irq() from macros to static inline functions if
!CONFIG_HIGHMEM, so we can easier detect mistakes like the one fixed in
93055c31

 ("ps3disk: passing wrong variable =
to
bvec_kunmap_irq()")

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>