IB/hfi1: Remove critical section gap in sc_buffer_alloc()

In sc_buffer_alloc(), the sc->alloc_lock is released
before calling sc_release_update(), and it is reacquired
after the function call. This causes CPU lock trading.
Fix it by not dropping the lock before calling
sc_release_update().

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>