IB/hfi1: Fix incorrect available receive user context count

The addition of the VNIC contexts to num_rcv_contexts changes the
meaning of the sysfs value nctxts from available user contexts, to
user contexts + reserved VNIC contexts.

User applications that use nctxts are now broken.

Update the calculation so that VNIC contexts are used only if there are
hardware contexts available, and do not silently affect nctxts.

Update code to use the calculated VNIC context number.

Update the sysfs value nctxts to be available user contexts only.

Fixes: 2280740f ("IB/hfi1: Virtual Network Interface Controller (VNIC) HW support")
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Niranjana Vishwanathapura <Niranjana.Vishwanathapura@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: <Stable@vger.kernel.org> #v4.12+
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>