ofp-actions: Fix use-after-free with ofpact_finish().

ofp-actions: Fix use-after-free with ofpact_finish().

ofpact_finish() may now reallocate the buffer it is passed, but not all
callers updated their local pointers to the current action in the
buffer. This could potentially lead to several use-after-free bugs.

Update ofpact_finish() to return the new pointer to the ofpact which is
provided, and update the calling points to ensure that their local
pointers are pointing into the correct (potentially reallocated) buffer.

Fixes: 2bd318dec242 ("ofp-actions: Make composing actions harder to screw up.")
Reported-by: William Tu <u9012063@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>