If this WARN_ON triggers it speaks to programmer error, and likely
implies corruption, but no released kernel should trigger it.  This
WARN_ON serves to assist DM integrity developers as changes are
made/tested in the future.

BUG_ON is excessive for catching programmer error, if a user or
developer would like warnings to trigger a panic, they can enable that
via /proc/sys/kernel/panic_on_warn
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>