be2iscsi: check ip buffer before copying

Dan Carpenter found a issue where be2iscsi would copy the ip
from userspace to the driver buffer before checking the len
of the data being copied:
http://marc.info/?l=linux-scsi&m=140982651504251&w=2

This patch just has us only copy what we the driver buffer
can support.

Cc: <stable@vger.kernel.org>
Tested-by: John Soni Jose <sony.john-n@emulex.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>