test: pass a large enough buffer to nvme_identify_ns_descs()

test: pass a large enough buffer to nvme_identify_ns_descs()

nvme_identify_ns_descs() takes a struct nvme_ns_id_desc * parameter,
but passes it as the data to nvme_identify(), which sets data_len = 4K.
But struct nvme_ns_id_desc only represents the start of a single
Namespace Identification Descriptor, so it is less than 4 KB.
So it needs to be explicitly allocated with at least 4 KB.
Allocate a 4 KB buffer in test.c to avoid a stack buffer overflow.

Signed-off-by: Caleb Sander <csander@purestorage.com>