datapath: Move is_frag out of struct ovs_skb_cb.

datapath: Move is_frag out of struct ovs_skb_cb.

is_frag is only used for communication between two functions, which
means that it doesn't really need to be in the SKB CB.  This wouldn't
necessarily be a problem except that there are also a number of other
paths that lead to this being uninitialized.  This isn't a problem
now but uninitialized memory seems dangerous and there isn't much
upside.

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