libceph: fall back to sendmsg for slab pages

skb_can_coalesce() allows coalescing neighboring slab objects into
a single frag:

  return page == skb_frag_page(frag) &&
         off == frag->page_offset + skb_frag_size(frag);

ceph_tcp_sendpage() can be handed slab pages.  One example of this is
XFS: it passes down sector sized slab objects for its metadata I/O.  If
the kernel client is co-located on the OSD node, the skb may go through
loopback and pop on the receive side with the exact same set of frags.
When tcp_recvmsg() attempts to copy out such a frag, hardened usercopy
complains because the size exceeds the object's allocated size:

  usercopy: kernel memory exposure attempt detected from ffff9ba917f20a00 (kmalloc-512) (1024 bytes)

Although skb_can_coalesce() could be taught to return false if the
resulting frag would cross a slab object boundary, we already have
a fallback for non-refcounted pages.  Utilize it for slab pages too.

Cc: stable@vger.kernel.org # 4.8+
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>