Move fib_alias out of fib_hash.c

["David S. Miller" <davem@xxxxxxxxxxxxx>]

Ok, this begins the movement of fib_alias object handling
out of fib_hash.c  The idea is to make fib_hash.c purely
a longest-matching-prefix lookup implementation.

Look at fn_hash_lookup() after these changes, and you'll
get an idea of how I want the whole fib_hash.c file to look
in the end.

I'll work on insert/delete tomorrow.

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