ofproto-dpif: Use a regular ref instead of try_ref for rule translation.

ofproto-dpif: Use a regular ref instead of try_ref for rule translation.

Until now, flow translation has had to use try_ref to take a reference on
a rule, because a competing thread might have released the last reference
and done an RCU-postponed deletion.  Since classifier versioning was
introduced, however, the release of the last reference is itself
RCU-postponed, which means that it is always safe to take the reference
directly.

Changing try_ref to ref means that taking a reference can't fail, which
allows the caller to take a reference in cases where the need to take a
reference was previously passed along a call chain, which simplifies some
code.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>