IB/ipath: Convert ipath_user_sdma_pin_pages() to use get_user_pages_fast()

ipath_user_sdma_queue_pkts() gets called with mmap_sem held for
writing.  Except for get_user_pages() deep down in
ipath_user_sdma_pin_pages() we don't seem to need mmap_sem at all.

Even more interestingly the function ipath_user_sdma_queue_pkts() (and
also ipath_user_sdma_coalesce() called somewhat later) call
copy_from_user() which can hit a page fault and we deadlock on trying
to get mmap_sem when handling that fault.  So just make
ipath_user_sdma_pin_pages() use get_user_pages_fast() and leave
mmap_sem locking for mm.

This deadlock has actually been observed in the wild when the node
is under memory pressure.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>

[ Merged in fix for call to get_user_pages_fast from Tetsuo Handa
  <penguin-kernel@I-love.SAKURA.ne.jp>.  - Roland ]
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/ipath/ipath_user_sdma.c

@@ -280,9 +280,7 @@ static int ipath_user_sdma_pin_pages(const struct ipath_devdata *dd,
 	int j;
 	int ret;
 
-	ret = get_user_pages(current, current->mm, addr,
-			     npages, 0, 1, pages, NULL);
-
+	ret = get_user_pages_fast(addr, npages, 0, pages);
 	if (ret != npages) {
 		int i;
 
@@ -811,10 +809,7 @@ int ipath_user_sdma_writev(struct ipath_devdata *dd,
 	while (dim) {
 		const int mxp = 8;
 
-		down_write(&current->mm->mmap_sem);
 		ret = ipath_user_sdma_queue_pkts(dd, pq, &list, iov, dim, mxp);
-		up_write(&current->mm->mmap_sem);
-
 		if (ret <= 0)
 			goto done_unlock;
 		else {