staging/rdma/hfi1: Change num_rcv_contexts to num_user_contexts and its meaning

num_rcv_contexts sets the number of user contexts, both receive and send.
Renaming it to num_user_contexts makes sense to reflect its true meaning.
When num_rcv_contexts is 0, the default behavior is the number of CPU
cores instead of 0 contexts. This commit changes the variable
num_rcv_contexts to num_user_contexts, and it also makes any negative
value for this variable default to the number of CPU cores, so if
num_user_contexts is set >= 0, the value will number of contexts.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>