rapidio: fix error handling path

rio_dma_transfer() attempts to clamp the return value of
pin_user_pages_fast() to be >= 0.  However, the attempt fails because
nr_pages is overridden a few lines later, and restored to the undesirable
-ERRNO value.

The return value is ultimately stored in nr_pages, which in turn is passed
to unpin_user_pages(), which expects nr_pages >= 0, else, disaster.

Fix this by fixing the nesting of the assignment to nr_pages: nr_pages
should be clamped to zero if pin_user_pages_fast() returns -ERRNO, or set
to the return value of pin_user_pages_fast(), otherwise.

[jhubbard@nvidia.com: new changelog]

Fixes: e8de370 ("rapidio: add mport char device driver")
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lkml.kernel.org/r/1600227737-20785-1-git-send-email-jrdr.linux@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Souptick Joarder

drivers/rapidio/devices/rio_mport_cdev.c

@@ -871,15 +871,16 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
 				rmcd_error("pin_user_pages_fast err=%ld",
 					   pinned);
 				nr_pages = 0;
-			} else
+			} else {
 				rmcd_error("pinned %ld out of %ld pages",
 					   pinned, nr_pages);
+				/*
+				 * Set nr_pages up to mean "how many pages to unpin, in
+				 * the error handler:
+				 */
+				nr_pages = pinned;
+			}
 			ret = -EFAULT;
-			/*
-			 * Set nr_pages up to mean "how many pages to unpin, in
-			 * the error handler:
-			 */
-			nr_pages = pinned;
 			goto err_pg;
 		}