datapath/flow_netlink: Create right mask with disabled megaflows

If megaflows are disabled, the userspace does not send the netlink attribute
OVS_FLOW_ATTR_MASK, and the kernel must create an exact match mask.

sw_flow_mask_set() sets every bytes (in 'range') of the mask to 0xff, even the
bytes that represent padding for struct sw_flow, or the bytes that represent
fields that may not be set during ovs_flow_extract().
This is a problem, because when we extract a flow from a packet,
we do not memset() anymore the struct sw_flow to 0 (since commit 9cef26ac6a71).

This commit gets rid of sw_flow_mask_set() and introduces mask_set_nlattr(),
which operates on the netlink attributes rather than on the mask key. Using
this approach we are sure that only the bytes that the user provided in the
flow are matched.

Also, if the parse_flow_mask_nlattrs() for the mask ENCAP attribute fails, we
now return with an error.

Reported-by: Alex Wang <alexw@nicira.com>
Suggested-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>