net: mpls: Fix GCC 12 warning

When building with automatic stack variable initialization, GCC 12
complains about variables defined outside of switch case statements.
Move the variable outside the switch, which silences the warning:

./net/mpls/af_mpls.c:1624:21: error: statement will never be executed [-Werror=switch-unreachable]
  1624 |                 int err;
       |                     ^~~

Signed-off-by: Victor Erminpour <victor.erminpour@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>