[SCSI] iscsi: only preallocate login buffer

We only use the mtask data buffer for login tasks so we do not
need to preallocate a buffer for every mtask. This saves
8 * 31 KB.

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