ima: ima/lsm policy rule loading logic bug fixes

Keep the ima policy rules around from the beginning even if they appear
invalid at the time of loading, as they may become active after an lsm
policy load.  However, loading a custom IMA policy with unknown LSM
labels is only safe after we have transitioned from the "built-in"
policy rules to a custom IMA policy.

Patch also fixes the rule re-use during the lsm policy reload and makes
some prints a bit more human readable.

Changelog:
v4:
- Do not allow the initial policy load refer to non-existing lsm rules.
v3:
- Fix too wide policy rule matching for non-initialized LSMs
v2:
- Fix log prints

Fixes: b1694245 ("ima: use the lsm policy update notifier")
Cc: Casey Schaufler <casey@schaufler-ca.com>
Reported-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Janne Karhunen <janne.karhunen@gmail.com>
Signed-off-by: Konsta Karsisto <konsta.karsisto@gmail.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>