Smack: fix typos and spelling errors

Fix typos and spelling errors in security/smack module comments that
were identified using the codespell tool.
No functional changes - documentation only.

Signed-off-by: Tanya Agarwal <tanyaagarwal25699@gmail.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>