btrfs: send: check for read-only send root under critical section

We're checking if the send root is read-only without being under the
protection of the root's root_item_lock spinlock, which is what protects
the root's flags when clearing the read-only flag, done at
btrfs_ioctl_subvol_setflags(). Furthermore, it should be done in the
same critical section that increments the root's send_in_progress counter,
as btrfs_ioctl_subvol_setflags() clears the read-only flag in the same
critical section that checks the counter's value.

So fix this by moving the read-only check under the critical section
delimited by the root's root_item_lock which also increments the root's
send_in_progress counter.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -8133,23 +8133,19 @@ long btrfs_ioctl_send(struct btrfs_inode *inode, const struct btrfs_ioctl_send_a
 		spin_unlock(&send_root->root_item_lock);
 		return -EPERM;
 	}
-	if (btrfs_root_readonly(send_root) && send_root->dedupe_in_progress) {
+	/* Userspace tools do the checks and warn the user if it's not RO. */
+	if (!btrfs_root_readonly(send_root)) {
+		spin_unlock(&send_root->root_item_lock);
+		return -EPERM;
+	}
+	if (send_root->dedupe_in_progress) {
 		dedupe_in_progress_warn(send_root);
 		spin_unlock(&send_root->root_item_lock);
 		return -EAGAIN;
 	}
 	send_root->send_in_progress++;
 	spin_unlock(&send_root->root_item_lock);
 
-	/*
-	 * Userspace tools do the checks and warn the user if it's
-	 * not RO.
-	 */
-	if (!btrfs_root_readonly(send_root)) {
-		ret = -EPERM;
-		goto out;
-	}
-
 	/*
 	 * Check that we don't overflow at later allocations, we request
 	 * clone_sources_count + 1 items, and compare to unsigned long inside