sdp: Limit skb frag size to 64K-1

sdp: Limit skb frag size to 64K-1

When 64K pages are in use, the skb_frag size can become larger
than the skb_frag can address. An skb_frag's max size is 64K-1.
This patch defines SDP_MAX_PAYLOAD as 64K - SDP_HEADER_SIZE.
The patch changes sdp_post_recv() and sdp_sendmsg() to use the smaller of
PAGE_SIZE or SDP_MAX_PAYLOAD as it segment size.

This fix the bug here:
https://bugs.openfabrics.org/show_bug.cgi?id=1300

Signed-off-by: David Wilder <dwilder@us.ibm.com>
Signed-off-by: Amir Vadai <amirv@mellanox.co.il>