drbd: get rid of bio_split, allow bios of "arbitrary" size

Where "arbitrary" size is currently 1 MiB, which is the BIO_MAX_SIZE
for architectures with 4k PAGE_CACHE_SIZE (most).

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>