IB/qib: Add sysfs interface to read free contexts

Indicate the number of free user contexts via the sysfs file
/sys/class/infiniband/qib0/nfreectxts as required for PSM.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>