block: hd: remove deprecated IRQF_DISABLED

This patch removes the use of the IRQF_DISABLED flag
from drivers/block/hd.c

It's a NOOP since 2.6.35 and it will be removed one day.

This also removes a related comment which is obsolete too.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Jens Axboe <axboe@fb.com>