datapath: Return correct error code when dumping flow actions.

Currently, if there isn't enough space to store the actions in a
flow during a dump we return -ENOMEM.  However, the standard error
in this situation is -EMSGSIZE so this changes the behavior to match.
This issue was introduced in 354d4c98a8cdaae3525848f564e58a9016bcd3af
 (datapath: Fix nelink attribute size for flow.).

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>