libceph: only call kernel_sendpage() via helper

libceph: only call kernel_sendpage() via helper

commit e36b13cceb46136d849aeee06b4907ad3570ba78 upstream.

Make ceph_tcp_sendpage() be the only place kernel_sendpage() is
used, by using this helper in write_partial_msg_pages().

Signed-off-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>
[bwh: Backported to 3.2:
 - Add ceph_tcp_sendpage(), from commit 31739139f3ed ('libceph: use
   kernel_sendpage() for sending zeroes'), the rest of which is not
   applicable
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>