btrfs: send: handle unexpected inode in header process_recorded_refs()

[ Upstream commit 5d228871 ]

Change BUG_ON to proper error handling when an unexpected inode number
is encountered. As the comment says this should never happen.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>