scsi: device_handler: remove VLAs

In preparation to enabling -Wvla, remove VLAs and replace them with
fixed-length arrays instead.

scsi_dh_{alua,emc,rdac} use variable-length array declarations to store
command blocks, with the appropriate size as determined by
COMMAND_SIZE. This patch replaces these with fixed-sized arrays using
MAX_COMMAND_SIZE, so that the array size can be determined at compile
time.

This was prompted by https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>