checkpatch: improve network block comment test and message

Show the first line of the comment after a line with just /* to better
show where the defective comment style is in the file.

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>