a) part of open_namei() done after we'd found vfsmount/dentry of
the object we want to open had been split into a helper - may_open().

	b) do_open() in fs/nfsctl.c didn't do any permission checks on
the nfsd file it was opening - sudden idiocy attack on my part (I missed
the fact that dentry_open() doesn't do permission checks - open_namei()
does).  Fixed by adding obvious may_open() calls.