[SCSI] iscsi class: fix gfp use in ping compl and host event

If a ping or host event were to occur when memory is low
we do not want to use GFP_KERNEL, because the paths
sending them cannot block for data to be written. These
paths might be needed to recover write paths. Use GFP_NOIO
instead.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>