lsm: add comment block for security_sk_classify_flow LSM hook

security_sk_classify_flow LSM hook has no comment block. Add a comment
block with a brief description of LSM hook and its function parameters.

Signed-off-by: Khadija Kamran <kamrankhadijadj@gmail.com>
[PM: minor double-space fix]
Signed-off-by: Paul Moore <paul@paul-moore.com>