cifs: Use file_dentry()

CIFS may be used as lower layer of overlayfs and accessing f_path.dentry can
lead to a crash.

Fix by replacing direct access of file->f_path.dentry with the
file_dentry() accessor, which will always return a native object.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>
diff --git a/fs/cifs/readdir.c b/fs/cifs/readdir.c
index 867439c..65cf85d 100644
--- a/fs/cifs/readdir.c
+++ b/fs/cifs/readdir.c
@@ -303,7 +303,7 @@
 	cifsFile->invalidHandle = true;
 	cifsFile->srch_inf.endOfSearch = false;
 
-	full_path = build_path_from_dentry(file->f_path.dentry);
+	full_path = build_path_from_dentry(file_dentry(file));
 	if (full_path == NULL) {
 		rc = -ENOMEM;
 		goto error_exit;
@@ -762,7 +762,7 @@
 		 */
 		fattr.cf_flags |= CIFS_FATTR_NEED_REVAL;
 
-	cifs_prime_dcache(file->f_path.dentry, &name, &fattr);
+	cifs_prime_dcache(file_dentry(file), &name, &fattr);
 
 	ino = cifs_uniqueid_to_ino_t(fattr.cf_uniqueid);
 	return !dir_emit(ctx, name.name, name.len, ino, fattr.cf_dtype);