efi: pstore: Drop efivar lock when efi_pstore_open() returns with an error

efi: pstore: Drop efivar lock when efi_pstore_open() returns with an error

If kzalloc fails, the function returns -ENOMEM without calling
efivar_unlock(). Since open() returned  an error, the calling site
in pstore_get_backend_records() won't call the close() function, so
the lock is never released. Thus drop the lock in case of errors here.

Fixes: 859748255b434 ("efi: pstore: Omit efivars caching EFI varstore access layer")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>