bcachefs: Change destroy_inode to free_inode

bcachefs: Change destroy_inode to free_inode

The vfs[1] documentation describes free_inode as follows:
```
free_inode
    this method is called from RCU callback. If you use call_rcu()
    in ->destroy_inode to free ‘struct inode’ memory, then it’s
    better to release memory in this method.
```

free_inode will be called by the RCU callback, so it might be better
to move the inode free operation to destroy_inode.

Similar to commit ae6b47b5653e ("fs/ntfs3: Change destroy_inode to
free_inode").

Link:
[1]: https://www.kernel.org/doc/html/latest/filesystems/vfs.html

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>