nvmet: use kzalloc instead of ZERO_PAGE in nvme_execute_identify_ns_nvm()

The nvme_execute_identify_ns_nvm function uses ZERO_PAGE for copying
SG list with all zeros. As ZERO_PAGE would not necessarily return the
virtual-address of the zero page, we need to first convert the page
address to kernel virtual-address and then use it as source address
for copying the data to SG list with all zeros. Using return address
of ZERO_PAGE(0) as source address for copying data to SG list would
fill the target buffer with random/garbage value and causes the
undesired side effect.

As other identify implemenations uses kzalloc for allocating a zero
filled buffer, we decided use kzalloc for allocating a zero filled
buffer in nvme_execute_identify_ns_nvm function and then use this
buffer for copying all zeros to SG list buffers. So esentially, we
now avoid using ZERO_PAGE.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Fixes: 64a51080 ("nvmet: implement id ns for nvm command set")
Link: https://lore.kernel.org/all/CAHj4cs8OVyxmn4XTvA=y4uQ3qWpdw-x3M3FSUYr-KpE-nhaFEA@mail.gmail.com/


Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>