ipv6: Refactor fib6_drop_pcpu_from

Move the existing pcpu walk in fib6_drop_pcpu_from to a new
helper, __fib6_drop_pcpu_from, that can be invoked per fib6_nh with a
reference to the from entries that need to be evicted. If the passed
in 'from' is non-NULL then only entries associated with that fib6_info
are removed (e.g., case where fib entry is deleted); if the 'from' is
NULL are entries are flushed (e.g., fib6_nh is deleted).

For fib6_info entries with builtin fib6_nh (ie., current code) there
is no change in behavior.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>