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>

net/ceph/messenger.c

@@ -278,7 +278,6 @@ static void _ceph_msgr_exit(void)
 	ceph_msgr_slab_exit();
 
 	BUG_ON(zero_page == NULL);
-	kunmap(zero_page);
 	page_cache_release(zero_page);
 	zero_page = NULL;
 }