scsi: libiscsi: fall back to sendmsg for slab pages

In "XFS over network block device" scenario XFS can create IO requests with
slab-based XFS metadata. During processing such requests tcp_sendpage() can
merge skb fragments with neighbour slab objects.

If receiving side is located on the same host tcp_recvmsg() can trigger
BUG_ON in hardening check and crash the host with following message:

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

This patch redirect such requests from sednpage to sendmsg path.  The
problem is similar to one described in recent commit 7e241f64
("libceph: fall back to sendmsg for slab pages")

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Acked-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>