IB/hfi1: Add a missing rcu_read_unlock()

This patch avoids that sparse reports the following:

drivers/infiniband/hw/hfi1/driver.c:251:13: warning: context imbalance in 'rcv_hdrerr' - different lock contexts for basic block

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>