btrfs: send: Ensure send_fd is writable

kernel_write() requires the caller to ensure that the file is writable.
Let's do that directly after looking up the ->send_fd.

(We don't need a separate bailout path because the "out" path already
does fput() if ->send_filp is non-NULL.)

fs/btrfs/send.c

@@ -8158,7 +8158,7 @@ long btrfs_ioctl_send(struct inode *inode, struct btrfs_ioctl_send_args *arg)
 	}
 
 	sctx->send_filp = fget(arg->send_fd);
-	if (!sctx->send_filp) {
+	if (!sctx->send_filp || !(sctx->send_filp->f_mode & FMODE_WRITE)) {
 		ret = -EBADF;
 		goto out;
 	}