[2/4] I/OAT: fix dma_pin_iovec_pages() error handling

Error handling needs to be modified in dma_pin_iovec_pages().
It should return NULL instead of ERR_PTR
(pinned_list is checked for NULL in tcp_recvmsg() to determine
if iovec pages have been successfully pinned down).
In case of error for the first iovec,
local_list->nr_iovecs needs to be initialized.

Cc: <stable@kernel.org>
Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>