libceph: use sock_no_sendpage() as a fallback in ceph_tcp_sendpage()

sock_no_sendpage() makes the code cleaner.

Also, don't set MSG_EOR.  sendpage doesn't act on MSG_EOR on its own,
it just honors the setting from the preceding sendmsg call by looking
at ->eor in tcp_skb_can_collapse_to().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>