btrfs: fix NULL dereference on root when tracing inode eviction

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

btrfs: fix NULL dereference on root when tracing inode eviction

When evicting an inode the first thing we do is to setup tracing for it,

which implies fetching the root's id. But in btrfs_evict_inode() the

root might be NULL, as implied in the next check that we do in

btrfs_evict_inode().

Hence, we either should set the ->root_objectid to 0 in case the root is

NULL, or we move tracing setup after checking that the root is not

NULL. Setting the rootid to 0 at least gives us the possibility to trace

this call even in the case when the root is NULL, so that's the solution

taken here.