security.h: fix misc typos/grammar errors in comments

Fix various typos/grammar errors in include/linux/security.h comments (no code changes).

Signed-off-by: Vsevolod Alekseev <vsevolod.alekseev@gmx.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>