Increase maximum accepted size for a DRBD meta dev

With the change to stripped LVs, the actual size of a meta device (which
is small) can be more than we expected (for non-stripped LVs). This
patch increases from 160MB to 1GB the accepted size, and updates the
comment with the rationale behind this change.

Note that we do want even meta devices stripped, since it can increase
metadata update.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>