rtnetlink: Move ifm in valid_fdb_dump_legacy to closer to use

Move setting of local variable ifm to after the message parsing in
valid_fdb_dump_legacy. Avoid potential future use of unchecked variable.

Fixes: 8dfbda19

 ("rtnetlink: Move input checking for rtnl_fdb_dump to helper")
Reported-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>