efi/cper: use stack buffer for error record decoding

Joe reports that using a statically allocated buffer for converting CPER
error records into human readable text is probably a bad idea. Even
though we are not aware of any actual issues, a stack buffer is clearly
a better choice here anyway, so let's move the buffer into the stack
frames of the two functions that refer to it.

Cc: <stable@vger.kernel.org>
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/cper.c

@@ -25,8 +25,6 @@
 #include <acpi/ghes.h>
 #include <ras/ras_event.h>
 
-static char rcd_decode_str[CPER_REC_LEN];
-
 /*
  * CPER record ID need to be unique even after reboot, because record
  * ID is used as index for ERST storage, while CPER records from
@@ -312,6 +310,7 @@ const char *cper_mem_err_unpack(struct trace_seq *p,
 				struct cper_mem_err_compact *cmem)
 {
 	const char *ret = trace_seq_buffer_ptr(p);
+	char rcd_decode_str[CPER_REC_LEN];
 
 	if (cper_mem_err_location(cmem, rcd_decode_str))
 		trace_seq_printf(p, "%s", rcd_decode_str);
@@ -326,6 +325,7 @@ static void cper_print_mem(const char *pfx, const struct cper_sec_mem_err *mem,
 	int len)
 {
 	struct cper_mem_err_compact cmem;
+	char rcd_decode_str[CPER_REC_LEN];
 
 	/* Don't trust UEFI 2.1/2.2 structure with bad validation bits */
 	if (len == sizeof(struct cper_sec_mem_err_old) &&