datapath: flow_netlink: Fix a bug.

Commit 62974663fe (datapath/flow_netlink: Create right mask with
disabled megaflows) introduced the bug which caused
ovs_nla_get_match() returns immediately after parsing the flow
mask for OVS_KEY_ATTR_ENCAP.  Consequently, when vlan encapsulated
packets are present, the corresponding datapath flows will have
incorrect mask like below.  And the incorrect flows could affect
other non-vlan packets.

~/ovs# ovs-dpctl dump-flows
in_port(3/0xffff0000),eth_type(0x8100),encap(), packets:0,
bytes:0, used:never, actions:2

This commit fixes the bug by checking and handling the return
value of the parsing function correctly.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>