Add assertion to catch 0-count page

Some network related block device uses tcp_sendpage, which doesn't
behave well when using 0-count page. Add assertion to catch them.

This has a runtime dependency on:
zfsonlinux/spl@ae16ed9 Fix crash when using ZFS on Ceph rbd

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2277