btrfs: fix invalid inode pointer dereferences during log replay

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix invalid inode pointer dereferences during log replay

In a few places where we call read_one_inode(), if we get a NULL pointer we end up jumping into an error path, or fallthrough in case of __add_inode_ref(), where we then do something like this:

iput(&inode->vfs_inode);

which results in an invalid inode pointer that triggers an invalid memory access, resulting in a crash.

Fix this by making sure we don't do such dereferences.