bpf: fix access to skb_shared_info->gso_segs

bpf: fix access to skb_shared_info->gso_segs

It is possible we reach bpf_convert_ctx_access() with
si->dst_reg == si->src_reg

Therefore, we need to load BPF_REG_AX before eventually
mangling si->src_reg.

syzbot generated this x86 code :
   3:   55                      push   %rbp
   4:   48 89 e5                mov    %rsp,%rbp
   7:   48 81 ec 00 00 00 00    sub    $0x0,%rsp // Might be avoided ?
   e:   53                      push   %rbx
   f:   41 55                   push   %r13
  11:   41 56                   push   %r14
  13:   41 57                   push   %r15
  15:   6a 00                   pushq  $0x0
  17:   31 c0                   xor    %eax,%eax
  19:   48 8b bf c0 00 00 00    mov    0xc0(%rdi),%rdi
  20:   44 8b 97 bc 00 00 00    mov    0xbc(%rdi),%r10d
  27:   4c 01 d7                add    %r10,%rdi
  2a:   48 0f b7 7f 06          movzwq 0x6(%rdi),%rdi // Crash
  2f:   5b                      pop    %rbx
  30:   41 5f                   pop    %r15
  32:   41 5e                   pop    %r14
  34:   41 5d                   pop    %r13
  36:   5b                      pop    %rbx
  37:   c9                      leaveq
  38:   c3                      retq

Fixes: d9ff286a0f59 ("bpf: allow BPF programs access skb_shared_info->gso_segs field")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>