tcp: Move code related to head frag in tcp_try_coalesce

This change reorders the code related to the use of an skb->head_frag so it
is placed before we check the rest of the frags.  This allows the code to
read more linearly instead of like some sort of loop.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>