efi-pstore: Refactor erase routine

Right now, every pass through the EFI variables during erase would build
a copy of the old format variable name. Instead, try each name one time
through the EFI variables list. Additionally bump up the buffer size to
avoid truncation in pathological cases, and wipe the write name buffer.

Signed-off-by: Kees Cook <keescook@chromium.org>