IB/hfi1: Remove duplicate definitions of num_user_pages() function

num_user_pages() function has been defined in both user_exp_rcv.c file
and user_sdma.c file. Move the function definition to a header file so
there is only one definition in the source repo.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>