vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/reiserfs/ioctl.c b/fs/reiserfs/ioctl.c
index 0b94d7b..950e3d1 100644
--- a/fs/reiserfs/ioctl.c
+++ b/fs/reiserfs/ioctl.c
@@ -96,7 +96,7 @@
 			inode->i_ctime = CURRENT_TIME_SEC;
 			mark_inode_dirty(inode);
 setflags_out:
-			mnt_drop_write(filp->f_path.mnt);
+			mnt_drop_write_file(filp);
 			break;
 		}
 	case REISERFS_IOC_GETVERSION:
@@ -117,7 +117,7 @@
 		inode->i_ctime = CURRENT_TIME_SEC;
 		mark_inode_dirty(inode);
 setversion_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		break;
 	default:
 		err = -ENOTTY;