[IPV4]: Fix garbage collection of multipath route entries

When garbage collecting route cache entries of multipath routes
in rt_garbage_collect(), entries were deleted from the hash bucket
'i' while holding a spin lock on bucket 'k' resulting in a system
hang.  Delete entries, if any, from bucket 'k' instead.

Signed-off-by: Suresh Bhogavilli <sbhogavilli@verisign.com>
Signed-off-by: David S. Miller <davem@davemloft.net>