ipv6: sit: Add ipip6_tunnel_dst_find() for cleanup

ipv6: sit: Add ipip6_tunnel_dst_find() for cleanup

Extract the dst lookup logic from ipip6_tunnel_xmit() into new helper
ipip6_tunnel_dst_find() to reduce code duplication and enhance readability.
No functional change intended.

On a x86_64, with allmodconfig object size is also reduced:

./scripts/bloat-o-meter net/ipv6/sit.o net/ipv6/sit-new.o
add/remove: 5/3 grow/shrink: 3/4 up/down: 1841/-2275 (-434)
Function                                     old     new   delta
ipip6_tunnel_dst_find                          -    1697   +1697
__pfx_ipip6_tunnel_dst_find                    -      64     +64
__UNIQUE_ID_modinfo2094                        -      43     +43
ipip6_tunnel_xmit.isra.cold                   79      88      +9
__UNIQUE_ID_modinfo2096                       12      20      +8
__UNIQUE_ID___addressable_init_module2092       -       8      +8
__UNIQUE_ID___addressable_cleanup_module2093       -       8      +8
__func__                                      55      59      +4
__UNIQUE_ID_modinfo2097                       20      18      -2
__UNIQUE_ID___addressable_init_module2093       8       -      -8
__UNIQUE_ID___addressable_cleanup_module2094       8       -      -8
__UNIQUE_ID_modinfo2098                       18       -     -18
__UNIQUE_ID_modinfo2095                       43      12     -31
descriptor                                   112      56     -56
ipip6_tunnel_xmit.isra                      9910    7758   -2152
Total: Before=72537, After=72103, chg -0.60%

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20250901114857.1968513-1-yuehaibing@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>