Revert "dm raid: remove unnecessary discard limits for raid10"

This reverts commit f0e90b6c

.

Matthew Ruffell reported data corruption in raid10 due to the changes
in discard handling [1]. Revert these changes before we find a proper fix.

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1907262/
Cc: Matthew Ruffell <matthew.ruffell@canonical.com>
Cc: Xiao Ni <xni@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>