[swss] advance sonic-swss sub module head (#2872)

Submodule src/sonic-swss ea4cba6..f8792d5:
  > [watermarkorch] only perform periodic clear if the polling is on (#781)
  > [arp] copy arp IO to cpu instead of trap and drop (#812)
  > fix bad parameter for gCrmOrch->incCrmAclUsedCounter in qosorch (#830)
  > [test_watermark] avoid watermark clear vs test random failure (#873)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>