IB/hfi1: Rename exp_lock to exp_mutex

The mutex exp_lock in struct hfi1_ctxtdata is used to protect all
Expected TID data of a user context. This patch renames it to exp_mutex
to better reflect its identity and prepare for upcoming patches.

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@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>