SDP: BUG1391 - bugs in the zero-copy send code

SDP: BUG1391 - bugs in the zero-copy send code

* fix sdp_bz_setup() code to handle the case of kernel data segment correctly (kernel sockets)
* make sdp_bz_setup() pass ENOMEM, EFAULT or other errors to sendmsg().
* Fix: the deallocation of bz descriptor in sendmsg() is not handled properly -- it is allocated many times, but freed once
* Fix: sdp_bzcopy_get() code does not raise reference count for all  pages in the bz descriptor (only the "partial" pages will get the count raised).
  However, the send completion code will call put_page() on all entries, leading to a crash for page-aligned transfers.

Signed-off-by: Constantine Gavrilov <constantine.gavrilov@gmail.com>
Signed-off-by: Amir Vadai <amirv@mellanox.co.il>