xen-netback: refactor xenvif_handle_frag_list()

When handling a from-guest frag list, xenvif_handle_frag_list()
replaces the frags before calling the destructor to clean up the
original (foreign) frags.  Whilst this is safe (the destructor doesn't
actually use the frags), it looks odd.

Reorder the function to be less confusing.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>