security,bpf: constify struct path in bpf_token_create() LSM hook

There is no reason why struct path pointer shouldn't be const-qualified
when being passed into bpf_token_create() LSM hook. Add that const.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>