checkpatch: allow control over line length warning, default remains 80

Some projects might want a longer line length so allow a command line
--max-line-length=n control over the long line warnings.  The default line
length is 80.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Constantine Shulyupin <const@makelinux.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>