do_make_slave(): choose new master sanely

When mount changes propagation type so that it doesn't propagate
events any more (MS_PRIVATE, MS_SLAVE, MS_UNBINDABLE), we need
to make sure that event propagation between other mounts is
unaffected.

We need to make sure that events from peers and master of that mount
(if any) still reach everything that used to be on its ->mnt_slave_list.

If mount has neither peers nor master, we simply need to dissolve
its ->mnt_slave_list and clear ->mnt_master of everything in there.

If mount has peers, we transfer everything in ->mnt_slave_list of
this mount into that of some of those peers (and adjust ->mnt_master
accordingly).

If mount has a master but no peers, we transfer everything in
->mnt_slave_list of this mount into that of its master (adjusting
->mnt_master, etc.).

There are two problems with the current implementation:
	* there's a long-obsolete logics in choosing the peer -
once upon a time it made sense to prefer the peer that had the
same ->mnt_root as our mount, but that had been pointless since
2014 ("smarter propagate_mnt()")
	* the most common caller of that thing is umount_tree()
taking the mounts out of propagation graph.  In that case it's
possible to have ->mnt_slave_list contents moved many times,
since the replacement master is likely to be taken out by the
same umount_tree(), etc.

Take the choice of replacement master into a separate function
(propagation_source()) and teach it to skip the candidates that
are going to be taken out.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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