fsverity: check IS_VERITY() in fsverity_cleanup_inode()

fsverity: check IS_VERITY() in fsverity_cleanup_inode()

Since getting the address of the fsverity_info has gotten a bit more
expensive, make fsverity_cleanup_inode() check for IS_VERITY() instead.
This avoids adding more overhead to non-verity files.

This assumes that verity info is never set when !IS_VERITY(), which is
currently true, but add a VFS_WARN_ON_ONCE() that asserts that.  (This
of course defeats the optimization, but only when CONFIG_VFS_DEBUG=y.)

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-14-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>