checkpatch, SubmittingPatches: suggest line wrapping commit messages at 75 columns

Commit messages lines are sometimes overly long.

Suggest line wrapping at 75 columns so the default git commit log
indentation of 4 plus the commit message text still fits on an 80 column
screen.

Add a checkpatch test for long commit messages lines too.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: David Miller <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ian Morris <ipm@chirality.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>