efi: pstore: Request at most 512 bytes for variable names

Work around a quirk in a few old (2011-ish) UEFI implementations, where
a call to `GetNextVariableName` with a buffer size larger than 512 bytes
will always return EFI_INVALID_PARAMETER.

This was already done to efivarfs in commit f45812cc

 ("efivarfs:
Request at most 512 bytes for variable names"), but the second copy of
the variable iteration implementation was overlooked.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>