icmp: fix icmp_ext_echo_iio parsing in icmp_build_probe

In icmp_build_probe(), the icmp_ext_echo_iio parsing should be done
step by step and skb_header_pointer() return value should always be
checked, this patch fixes 3 places in there:

  - On case ICMP_EXT_ECHO_CTYPE_NAME, it should only copy ident.name
    from skb by skb_header_pointer(), its len is ident_len. Besides,
    the return value of skb_header_pointer() should always be checked.

  - On case ICMP_EXT_ECHO_CTYPE_INDEX, move ident_len check ahead of
    skb_header_pointer(), and also do the return value check for
    skb_header_pointer().

  - On case ICMP_EXT_ECHO_CTYPE_ADDR, before accessing iio->ident.addr.
    ctype3_hdr.addrlen, skb_header_pointer() should be called first,
    then check its return value and ident_len.
    On subcases ICMP_AFI_IP and ICMP_AFI_IP6, also do check for ident.
    addr.ctype3_hdr.addrlen and skb_header_pointer()'s return value.
    On subcase ICMP_AFI_IP, the len for skb_header_pointer() should be
    "sizeof(...