mount: add helper to check unsupported children collision

See more detailed explanation inside in-code comment.

note: Actually before we remove validate_mounts (later in these
patchset) we likely won't get to these check and fail earlier, as having
children collision implies shared mounts with different sets of
children.

note: from v4.11 and ms kernel commit 1064f874abc0 ("mnt: Tuck mounts
under others instead of creating shadow/side mounts.") there will be no
more mount collision.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>