ipv6: exthdrs: Reload hdr only when needed in ipv6_srh_rcv().

We need not reload hdr in ipv6_srh_rcv() unless we call
pskb_expand_head().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>