bpf: Update local storage test to check handling of null ptrs

It was found in [1] that bpf_inode_storage_get helper did not check
the nullness of the passed owner ptr which caused an oops when
dereferenced. This change incorporates the example suggested in [1] into
the local storage selftest.

The test is updated to create a temporary directory instead of just
using a tempfile. In order to replicate the issue this copied rm binary
is renamed tiggering the inode_rename with a null pointer for the
new_inode. The logic to verify the setting and deletion of the inode
local storage of the old inode is also moved to this LSM hook.

The change also removes the copy_rm function and simply shells out
to copy files and recursively delete directories and consolidates the
logic of setting the initial inode storage to the bprm_committed_creds
hook and removes the file_open hook.

[1]: https://lore.kernel.org/bpf/CANaYP3HWkH91SN=wTNO9FL_2ztHfqcXKX38SSE-JJ2voh+vssw@mail.gmail.com

Suggested-by: Gilad Reti <gilad.reti@gmail.com>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210112075525.256820-2-kpsingh@kernel.org