ext4: fix invalid free tracking in ext4_xattr_move_to_block()

In ext4_xattr_move_to_block(), the value of the extended attribute
which we need to move to an external block may be allocated by
kvmalloc() if the value is stored in an external inode. So at the end
of the function the code tried to check if this was the case by
testing entry->e_value_inum.

However, at this point, the pointer to the xattr entry is no longer
valid, because it was removed from the original location where it had
been stored. So we could end up calling kvfree() on a pointer which
was not allocated by kvmalloc(); or we could also potentially leak
memory by not freeing the buffer when it should be freed. Fix this by
storing whether it should be freed in a separate variable.

Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Link: https://syzkaller.appspot.com/bug?id=5c2aee8256e30b55ccf57312c16d88417adbd5e1
Link: https://syzkaller.appspot.com/bug?id=41a6b5d4917c0412eb3b3c3c604965bed7d7420b
Reported-by: [e-mail]
Reported-by: [e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/xattr.c

···
 		.in_inode = !!entry->e_value_inum,
 	};
 	struct ext4_xattr_ibody_header *header = IHDR(inode, raw_inode);
+	int needs_kvfree = 0;
 	int error;
 
 	is = kzalloc(sizeof(struct ext4_xattr_ibody_find), GFP_NOFS);
···
 			error = -ENOMEM;
 			goto out;
 		}
-
+		needs_kvfree = 1;
 		error = ext4_xattr_inode_get(inode, entry, buffer, value_size);
 		if (error)
 			goto out;
···
 
 out:
 	kfree(b_entry_name);
-	if (entry->e_value_inum && buffer)
+	if (needs_kvfree && buffer)
 		kvfree(buffer);
 	if (is)
 		brelse(is->iloc.bh);