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>