datapath: Avoid using stack larger than 1024.

commit (datapath: Refactor action alloc and copy api) effectively
reverted 1d2a1b5f5252e4c6ce8bbf8d91ca27aba52496e6 (datapath: Factor out
allocation and verification of actions.). This results in following
warning:

CC [M]  /home/jesse/openvswitch/datapath/linux/datapath.o
/home/jesse/openvswitch/datapath/linux/datapath.c: In function
â€˜ovs_flow_cmd_setâ€™:
/home/jesse/openvswitch/datapath/linux/datapath.c:1094:1: warning: the
frame size of 1256 bytes is larger than 1024 bytes
[-Wframe-larger-than=]
 }

Following patch introduced the factoring back.

CC: Jesse Gross <jesse@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>