libceph: Remove spurious kunmap() of the zero page

ceph_tcp_sendpage already does the work of mapping/unmapping
the zero page if needed.

Signed-off-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>