datapath: Avoid NULL mask check while building mask

OVS does mask validation even if it does not need to convert
netlink mask attributes to mask structure.  ovs_nla_get_match()
caller can pass NULL mask structure pointer if the caller does
not need mask.  Therefore NULL check is required in SW_FLOW_KEY*
macros.  Following patch does not convert mask netlink attributes
if mask pointer is NULL, so we do not need these checks in
SW_FLOW_KEY* macro.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Andy Zhou <azhou@nicira.com>