fsverity fixup

After freeing i_verity_info, it's necessary to set it NULL since the
the inode structure doesn't get cleared when it gets allocated.
That's why it's up to functions like inode_init_always() to reset
various fields to be zero.  F or i_verity_info, we should do that in
fsverity_cleanup_inode().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>