ipv6: Fix bug in ipv6_chk_same_addr().

hlist_for_each_entry(p...) will not necessarily initialize 'p'
to anything if the hlist is empty.  GCC notices this and emits
a warning.

Just return true explicitly when we hit a match, and return
false is we fall out of the loop without one.

Signed-off-by: David S. Miller <davem@davemloft.net>