iommu/arm-smmu-v3: Stop pre-zeroing batch commands

iommu/arm-smmu-v3: Stop pre-zeroing batch commands

Pre-zeroing the batched commands structure is inefficient, as individual
commands are zeroed later in arm_smmu_cmdq_build_cmd(). The size is quite
large and commonly most commands won't even be used:

	struct arm_smmu_cmdq_batch cmds = {};
345c:	52800001 	mov	w1, #0x0                   	// #0
3460:	d2808102 	mov	x2, #0x408                 	// #1032
3464:	910143a0 	add	x0, x29, #0x50
3468:	94000000 	bl	0 <memset>

Stop pre-zeroing the complete structure and only zero the num member.

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1628696966-88386-1-git-send-email-john.garry@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>