pstore/ram: Avoid needless alloc during header write

static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz,

				     struct pstore_record *record)

{

	char *hdr;

	char hdr[36]; /* "===="(4), %lld(20), "."(1), %06lu(6), "-%c\n"(3) */

	size_t len;

	hdr = kasprintf(GFP_ATOMIC, RAMOOPS_KERNMSG_HDR "%lld.%06lu-%c\n",

	len = scnprintf(hdr, sizeof(hdr),

		RAMOOPS_KERNMSG_HDR "%lld.%06lu-%c\n",

		(time64_t)record->time.tv_sec,

		record->time.tv_nsec / 1000,

		record->compressed ? 'C' : 'D');

	if (WARN_ON_ONCE(!hdr))

		return 0;

	len = strlen(hdr);

	persistent_ram_write(prz, hdr, len);

	kfree(hdr);

	return len;

}