nbd: fix 64-bit division

commit e88f72cb9f54f6d244e55f629fe5e2f34ca6f9ed upstream.

We have this:

ERROR: "__aeabi_ldivmod" [drivers/block/nbd.ko] undefined!
ERROR: "__divdi3" [drivers/block/nbd.ko] undefined!
nbd.c:(.text+0x247c72): undefined reference to `__divdi3'

due to a recent commit, that did 64-bit division. Use the proper
divider function so that 32-bit compiles don't break.

Fixes: ef77b515243b ("nbd: use loff_t for blocksize and nbd_set_size args")
Signed-off-by: Jens Axboe <axboe@fb.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>