IB/hfi1: Add pin query function

System administrators can use the locked memory
ulimit setting to set the maximum amount of memory
a user can lock/pin. However, this setting alone is not
enough to guarantee good operation of the hfi1 driver
due to the fact that the setting does not have fine
enough granularity to account for the limit being used
by multiple user processes and caches.

Therefore, a better limiting algorithm is needed. This
is where the new hfi1_can_pin_pages() function and the
cache_size module parameter come in.

The function works by looking at the ulimit and cache_size
value to compute a cache size. The algorithm examines the
ulimit value and, if it is not "unlimited", computes a
per-cache limit based on the number of configured user
contexts.

After that, the lower of the two - cache_size and computed
per-cache limit - is used.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>