efi-pstore: Fix write/erase id tracking

Prior to the pstore interface refactoring, the "id" generated during
a backend pstore_write() was only retained by the internal pstore
inode tracking list. Additionally the "part" was ignored, so EFI
would encode this in the id. This corrects the misunderstandings
and correctly sets "id" during pstore_write(), and uses "part"
directly during pstore_erase().

Reported-by: Marta Lofstedt <marta.lofstedt@intel.com>
Fixes: 76cc9580 ("pstore: Replace arguments for write() API")
Fixes: a61072aa ("pstore: Replace arguments for erase() API")
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Marta Lofstedt <marta.lofstedt@intel.com>