[PATCHES][CFR] vfs fixes

[Al Viro <viro@zeniv.linux.org.uk>]

	Fixes for assorted bugs caught by struct mount audit.
This stuff sits in
git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #fixes

Please, review; I'm going to push those to Linus in a few days.

Individual patches in followups.

1) fs/fhandle.c: fix a race in call of has_locked_children()
	traversing the list of children without mount_lock; oopsable,
present since v6.11.
2) path_overmount(): avoid false negatives
	namespace_sem is not enough to prevent false negatives from
__lookup_mnt(); rcu_read_lock() makes it memory-safe, but mount_lock
seqretry is needed for valid result.  Present since _way_ back -
predates path_overmount(), actually.  Originally introduced in v5.7
3) finish_automount(): don't leak MNT_LOCKED from parent to child
	MNT_LOCKED is incompatible with MNT_SHRINKABLE and such
combinations had been prevented from the very beginning; unfortunately,
one case got missed - automount triggered within an MNT_LOCKED mount.
Goes all the way back to v3.12...
4) fix propagation graph breakage by MOVE_MOUNT_SET_GROUP move_mount(2)
	In case when old mount both receives and transmits mount events,
do_set_group() end up corrupting the data structures.  Introduced in
v5.15
5) fs: allow clone_private_mount() for a path on real rootfs
	v6.15 introduced a way to use locations in detached
trees as overlayfs layers; unfortunately, the way it had
been done ended up breaking something that used to be allowed -
using locations on initramfs as overlayfs layers.  Turns out
that people really used such setups...