vfs: Don't allow changing the link count of an inode with an invalid uid or gid

Changing the link count of an inode via unlink or link will cause a
write back of that inode.  If the uids or gids are invalid (aka not known
to the kernel) writing the inode back may change the uid or gid in the
filesystem.   To prevent possible filesystem and to avoid the need for
filesystem maintainers to worry about it don't allow operations on
inodes with an invalid uid or gid.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>