nvme-fabrics: use consistent zeroout pattern

Remove zeroout memeset call & zeroout local variable cmd at the time
of declaration in nvmf_ref_read32() similar to what we have done in
nvmf_reg_read64(), nvmf_reg_write32(), nvmf_connect_admin_queue(), and
nvmf_connect_io_queue().

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>