[SCSI] ibmvscsi: avoid unnecessary use of kzalloc_pool

The allocated struct is manually zeroed after allocation, so avoid using
the (broken) kzalloc mempool (which does not re-zero previously used items
when they are returned to the pool).

Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>