finish_automount(): don't leak MNT_LOCKED from parent to child

Intention for MNT_LOCKED had always been to protect the internal
mountpoints within a subtree that got copied across the userns boundary,
not the mountpoint that tree got attached to - after all, it _was_
exposed before the copying.

For roots of secondary copies that is enforced in attach_recursive_mnt() -
MNT_LOCKED is explicitly stripped for those.  For the root of primary
copy we are almost always guaranteed that MNT_LOCKED won't be there,
so attach_recursive_mnt() doesn't bother.  Unfortunately, one call
chain got overlooked - triggering e.g. NFS referral will have the
submount inherit the public flags from parent; that's fine for such
things as read-only, nosuid, etc., but not for MNT_LOCKED.

This is particularly pointless since the mount attached by finish_automount()
is usually expirable, which makes any protection granted by MNT_LOCKED
null and void; just wait for a while and that mount will go away on its own.

Include MNT_LOCKED into the set of flags to be ignored by do_add_mount() - it
really is an internal flag.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Fixes: 5ff9d8a6 ("vfs: Lock in place mounts from more privileged users")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>