checkpatch: add fix_insert_line and fix_delete_line helpers

Neaten the uses of patch/file line insertions or deletions.  Hide the
mechanism used.

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>