ima: Implement ima_inode_hash

This is in preparation to add a helper for BPF LSM programs to use
IMA hashes when attached to LSM hooks. There are LSM hooks like
inode_unlink which do not have a struct file * argument and cannot
use the existing ima_file_hash API.

An inode based API is, therefore, useful in LSM based detections like an
executable trying to delete itself which rely on the inode_unlink LSM
hook.

Moreover, the ima_file_hash function does nothing with the struct file
pointer apart from calling file_inode on it and converting it to an
inode.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20201124151210.1081188-2-kpsingh@chromium.org