Correctly check share permissions when updating a re-sub-share

Before this change the node you shared was checked for permissions.
This works when you reshare the folder that was shared with you.
However when you reshared a subfolder (e.g. as public link),
you could afterwards update the permissions and grant
create+update permissions although the share you receive was read-only.

Signed-off-by: Joas Schilling <coding@schilljs.com>