learn: Fix iteration over learning specs.

learn: Fix iteration over learning specs.

struct ofpact_learn_spec is variable-length.  The 'n_specs' member of
struct ofpact_learn counted the number of specs, but the iteration loops
over struct ofpact_learn_spec only iterated as far as the *minimum* length
of 'n_specs' specs.

This fixes the problem, which exhibited as consistent failures for test 431
(learning action - TCPv6 port learning), seemingly only on i386 since it
shows up for my personal development machine but appears to not happen for
anyone else.

Fixes: dfe191d5faa6 ("ofp-actions: Waste less memory in learn actions.")
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>