[PATCH] use SK_MEM_QUANTUM_SHIFT in __sk_mem_reclaim()

Avoid an expensive divide (as done in commit
18030477e70a826b91608aee40a987bbd368fec6 but lost in commit
23821d2653111d20e75472c8c5003df1a55309a8)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>