ip: introduce ip_is_fragment helper inline function

There are enough instances of this:

    iph->frag_off & htons(IP_MF | IP_OFFSET)

that a helper function is probably warranted.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
net: Fix build failures due to ip_is_fragment()

It needs to be available even when CONFIG_INET is not set.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>