mkfs: warn about V4 deprecation when creating new V4 filesystems

mkfs: warn about V4 deprecation when creating new V4 filesystems

The V4 filesystem format is deprecated in the upstream Linux kernel.  In
September 2025 it will be turned off by default in the kernel and five
years after that, support will be removed entirely.  Warn people
formatting new filesystems with the old format, particularly since V4 is
not the default.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>