pnfsblock: fix NULL pointer dereference

bl_add_page_to_bio returns error pointer. bio should be reset to
NULL in failure cases as the out path always calls bl_submit_bio.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Cc: stable@kernel.org [3.0]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/blocklayout/blocklayout.c

@@ -292,6 +292,7 @@ bl_read_pagelist(struct nfs_read_data *rdata)
 						 bl_end_io_read, par);
 			if (IS_ERR(bio)) {
 				rdata->pnfs_error = PTR_ERR(bio);
+				bio = NULL;
 				goto out;
 			}
 		}
@@ -581,6 +582,7 @@ fill_invalid_ext:
 						 bl_end_io_write_zero, par);
 			if (IS_ERR(bio)) {
 				wdata->pnfs_error = PTR_ERR(bio);
+				bio = NULL;
 				goto out;
 			}
 			/* FIXME: This should be done in bi_end_io */
@@ -629,6 +631,7 @@ next_page:
 					 bl_end_io_write, par);
 		if (IS_ERR(bio)) {
 			wdata->pnfs_error = PTR_ERR(bio);
+			bio = NULL;
 			goto out;
 		}
 		isect += PAGE_CACHE_SECTORS;