ANDROID: sdcardfs: Set num in extension_details during make_item

Without this patch when you delete an extension from configfs
it still exists in the hash table data structures and we are
unable to delete it or change it's group.
This happens because during deletion the key & value is taken from
extension_details, and was not properly set.

Fix it by this patch.

Change-Id: I7c20cb1ab4d99e6aceadcb5ef850f0bb47f18be8
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 73055997