[FIB]: Avoid using static variables without proper locking

fib_trie_seq_show() uses two helper functions, rtn_scope() and
rtn_type() that can write to static storage without locking.

Just pass to them a temporary buffer to avoid potential corruption
(probably not triggerable but still...)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>