block: Add bio_alloc_pages()

More utility code to replace stuff that's getting open coded.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
CC: NeilBrown <neilb@suse.de>

diff --cc drivers/md/raid1.c
Simple merge

diff --cc fs/bio.c
Simple merge

diff --cc include/linux/bio.h
Simple merge