btrfs: fix use-after-free on inode when scanning root during em shrinking

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

btrfs: fix use-after-free on inode when scanning root during em shrinking

At btrfs_scan_root() we are accessing the inode's root (and fs_info) in a

call to btrfs_fs_closing() after we have scheduled the inode for a delayed

iput, and that can result in a use-after-free on the inode in case the

cleaner kthread does the iput before we dereference the inode in the call

to btrfs_fs_closing().

Fix this by using the fs_info stored already in a local variable instead

of doing inode->root->fs_info.