staging/hfi1: Convert lock to mutex

The exp_lock lock does not need to be a spinlock as
all its uses are in process context and allowing the
process to sleep when the mutex is contended might
be beneficial.

Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>