datapath: Free up flow_extract() return value for reporting errors.

datapath: Free up flow_extract() return value for reporting errors.

flow_extract() can fail due to memory allocation errors in pskb_may_pull().
Currently it doesn't return those properly, instead just reporting a bogus
flow to the caller.  But its return value is currently in use for reporting
whether the packet was an IPv4 fragment.  This commit switches to reporting
that in the skb itself so that the return value can be reused to report
errors.

Signed-off-by: Ben Pfaff <blp@nicira.com>