fsverity: RCU-delay the freeing of struct fsverity_info

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

fsverity: RCU-delay the freeing of struct fsverity_info

__fsverity_get_info() calls rhashtable_lookup_fast(), which just uses
rcu_read_lock() and doesn't directly synchronize with
fsverity_remove_info().

For the same inode this isn't a problem: its fsverity_info is removed
only at inode eviction time or upon failure to enable verity, when the
inode no longer needs its fsverity_info and it will no longer be
accessed via that inode.

However, this is broken and can cause a use-after-free for concurrent
__fsverity_get_info() for *different* inodes. Those rely on following
fsverity_info::rhash_head in the rhashtable under rcu_read_lock() only.
They also use fsverity_info::inode to do the key comparison.

Fix this by RCU-delaying the freeing of 'struct fsverity_info' after
it's been removed from the rhashtable.