libceph: fix corruption when using page_count 0 page in rbd

It has been reported that using ZFSonLinux on rbd will result in memory
corruption. The bug report can be found here:

https://github.com/zfsonlinux/spl/issues/241
http://tracker.ceph.com/issues/7790

The reason is that ZFS will send pages with page_count 0 into rbd, which in
turns send them to tcp_sendpage. However, tcp_sendpage cannot deal with
page_count 0, as it will do get_page and put_page, and erroneously free the
page.

This type of issue has been noted before, and handled in iscsi, drbd,
etc. So, rbd should also handle this. This fix address this issue by fall back
to slower sendmsg when page_count 0 detected.

Cc: Sage Weil <sage@inktank.com>
Cc: Yehuda Sadeh <yehuda@inktank.com>
Cc: stable@vger.kernel.org
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Ilya Dryomov <ilya.dryomov@inktank.com>