modules: Add kernel parameter to blacklist modules

Blacklisting a module in linux has long been a problem.  The current
procedure is to use rd.blacklist=module_name, however, that doesn't
cover the case after the initramfs and before a boot prompt (where one
is supposed to use /etc/modprobe.d/blacklist.conf to blacklist
runtime loading). Using rd.shell to get an early prompt is hit-or-miss,
and doesn't cover all situations AFAICT.

This patch adds this functionality of permanently blacklisting a module
by its name via the kernel parameter module_blacklist=module_name.

[v2]: Rusty, use core_param() instead of __setup() which simplifies
things.

[v3]: Rusty, undo wreckage from strsep()

[v4]: Rusty, simpler version of blacklisted()

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 724970a..c59ae1a 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2301,6 +2301,9 @@
 			Note that if CONFIG_MODULE_SIG_FORCE is set, that
 			is always true, so this option does nothing.
 
+	module_blacklist=  [KNL] Do not load a comma-separated list of
+			modules.  Useful for debugging problem modules.
+
 	mousedev.tap_time=
 			[MOUSE] Maximum time between finger touching and
 			leaving touchpad surface for touch to be considered