selinux: checkreqprot is deprecated, add some ssleep() discomfort

The checkreqprot functionality was disabled by default back in
Linux v4.4 (2015) with commit 2a35d196 ("selinux: change
CONFIG_SECURITY_SELINUX_CHECKREQPROT_VALUE default") and it was
officially marked as deprecated in Linux v5.7.  It was always a
bit of a hack to workaround very old userspace and to the best of
our knowledge, the checkreqprot functionality has been disabled by
Linux distributions for quite some time.

This patch moves the deprecation messages from KERN_WARNING to
KERN_ERR and adds a five second sleep to anyone using it to help
draw their attention to the deprecation and provide a URL which
helps explain things in more detail.

Signed-off-by: Paul Moore <paul@paul-moore.com>