checkpatch: rename variables to avoid confusion

The variable name "$msg_type" is sometimes used to set the message type,
and sometimes used to set the message level.  This works but is kind of
confusing.  Use "$msg_level" in the latter case instead, to make the code
clearer.

Link: http://lkml.kernel.org/r/20170902175345.175db33a@endymion


Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>