cache: support no_discard_passdown

cache: support no_discard_passdown

Recent kernel version from kernel commit:
de7180ff908b2bc0342e832dbdaa9a5f1ecaa33a
started to report in cache status line new flag:
no_discard_passdown

Whenever lvm spots unknown status it reports:
Unknown feature in status:

So add reconginzing this feature flag and also report this with

'lvs -o+kernel_discards'

When no_discard_passdown is found in status 'nopassdown' gets reported
for this field  (roughly matching what we report for thin-pools).