mds: add root_squash
Allow a root squash mode via the MDS capability. The purpose here is not so much to prevent a true adversary (the client always send a request as the appropriate uid/gid), but instead to prevent an accidental command like, say, rm -r $PTAH/ (where $PATH is presumably something real but $PTAH is not).
"CERN was asking for something even simpler: allow hosts to mount and interact as any user, except prevent root. The specific use-case is to avoid an accidental 'sudo rm -rf ...'.
I think this would take the form of a flag on the normal grant object where root_squash=true. Requests with any uid != 0 would be permitted, but uid == 0 denied. Obviously a malicious user could simply delete each file as the uid the file is owned by, but the purpose of the flag is avoiding user error, not security."