IB/hfi1: IB/hfi1: Add an API to handle special case drop

IB/hfi1: IB/hfi1: Add an API to handle special case drop

This patch pushes special case drop logic into an API to be shared by all
interrupt handlers.

Additionally, convert do_drop to a bool.

Link: https://lore.kernel.org/r/20200106134203.119356.36962.stgit@awfm-01.aw.intel.com
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>