checkpatch: fix regressions in "fix handling of leading spaces"

The patch "checkpatch: fix handling of leading spaces" added checks for
leading spaces on lines, but this introduces regressions.  Firstly it does
not correctly detect when we are in a comment.  Secondly it does not allow
for preprocessor command spacing.  Finally it does not allow for label
indentation which is required to be less than one tab.  Fix these up:

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>