hfi1/user_exp_rcv: rework main loop

New loop now takes into account pages not physically contiguous.
Also some minor improvements, e.g. make the spin_lock used more locally,
reuse a group we had if we had one, etc.

kernel/include/hfi1/chip.h

@@ -51,6 +51,8 @@
  * This file contains all of the defines that is specific to the HFI chip
  */
 
+#define MAX_EXPECTED_BUFFER    (2048 * 1024)
+
 void hfi1_put_tid(struct hfi1_devdata *dd, u32 index,
                   u32 type, unsigned long pa, u16 order);
 void hfi1_clear_tids(struct hfi1_ctxtdata *rcd);