[FIB]: Reduce text size of net/ipv4/fib_trie.o

[FIB]: Reduce text size of net/ipv4/fib_trie.o

In struct tnode, we use two fields of 5 bits for 'pos' and 'bits'.
Switching to plain 'unsigned char' (8 bits) take the same space
because of compiler alignments, and reduce text size by 435 bytes
on i386.

On i386 :
$ size net/ipv4/fib_trie.o.before_patch net/ipv4/fib_trie.o
    text    data     bss     dec     hex filename
   13714       4      64   13782    35d6 net/ipv4/fib_trie.o.before
   13279       4      64   13347    3423 net/ipv4/fib_trie.o

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>