Btrfs: assert send doesn't attempt to start transactions

When starting a transaction just assert that current->journal_info
doesn't contain a send transaction stub, since send isn't supposed
to start transactions and when it finishes (either successfully or
not) it's supposed to set current->journal_info to NULL.

This is motivated by the change titled:

    Btrfs: fix crash when starting transaction
Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/transaction.c

@@ -386,11 +386,13 @@ start_transaction(struct btrfs_root *root, u64 num_items, unsigned int type,
 	bool reloc_reserved = false;
 	int ret;
 
+	/* Send isn't supposed to start transactions. */
+	ASSERT(current->journal_info != (void *)BTRFS_SEND_TRANS_STUB);
+
 	if (test_bit(BTRFS_FS_STATE_ERROR, &root->fs_info->fs_state))
 		return ERR_PTR(-EROFS);
 
-	if (current->journal_info &&
-	    current->journal_info != (void *)BTRFS_SEND_TRANS_STUB) {
+	if (current->journal_info) {
 		WARN_ON(type & TRANS_EXTWRITERS);
 		h = current->journal_info;
 		h->use_count++;