RDMA/vmw_pvrdma: Dont hardcode QP header page

Moved the header page count to a macro.

Reported-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Adit Ranadive <aditr@vmware.com>
Reviewed-by: Aditya Sarwade <asarwade@vmware.com>
Tested-by: Andrew Boyer <andrew.boyer@dell.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>