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>