ipv6: Clean up ipv6_select_ident() and ip6_fragment()

This patch changes the ipv6_select_ident() signature to return a
fragment id instead of taking a whole frag_hdr as a param to
only set the frag_hdr->identification.

It also cleans up ip6_fragment() to obtain the fragment id at the
beginning instead of using multiple "if" later to check fragment id
has been generated or not.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>