Revert "dm raid: fix discard limits for raid1 and raid10"

This reverts commit e0910c8e.

Reverting 6ffeb1c3 ("md: change mddev 'chunk_sectors' from int to
unsigned") exposes dm-raid.c compiler warnings detailed that commit's
header. Clearly this more conservative fix, of simply reverting
e0910c8e, would've been more prudent given how late we were in the
v5.10 release. Lessons have been learned.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>