checkpatch: reduce runtime/cpu time used

There are some cases where checkpatch can take a long time to complete.
Reduce the likelihood of this long run-time by adding a new test for lines
with and without comments and eliminating checks on lines with only
comments.

This reduces the number of "ctx_statement_block" calls, and also the
number of tests of $stat, which is now undefined for these blank lines.

One test in particular, the "check for switch/default statements without a
break", could take an extremely long time to parse as it tries to skip
interleaving comments within the ctx_statement_block/$stat and that could
be done multiple times unnecessarily.

A small test case taken from cfg80211.h before this patch would take
1000's of seconds to run, now it's just a couple seconds.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>