dpif-linux: Fix revalidator core caused by flow dumps.

The dpif flow dump API guarantees that keys it returns are not deleted
unless dpif_flow_dump_next_may_destroy_keys() warns that they might
be.  When dpif_linux_flow_dump_next() needed extra memory for a
datapath flow's actions, it would use a special temporary buffer.
Unfortunately, it also used memory from this temporary buffer for the
keys it returned.  Thus, on the next loop this memory would be freed,
breaking our invariant.

The correct solution to this problem is probably to fix this rather
awkward API.  However, this solution is small and simple, so it's fine
for now.

Signed-off-by: Ethan Jackson <ethan@nicira.com>