Ensure that gang_copies is always at least as large as copies

As discussed in the comments of PR #17004, you can theoretically run
into a case where a gang child has more copies than the gang header,
which can lead to some odd accounting behavior (and even trip a
VERIFY). While the accounting code could be changed to handle this, it
fundamentally doesn't seem to make a lot of sense to allow this to
happen. If the data is supposed to have a certain level of reliability,
that isn't actually achieved unless the gang_copies property is set to
match it.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Closes #17484