scripts/checkpatch.pl: some improvements to reduce false positives

scripts/checkpatch.pl: some improvements to reduce false positives

- camelcase is OK for printk inttypes.h;
- strncpy is OK;
- accept up to 120 chars on lines without warnings;
- stop complaining about "BACKTRACE=" strings split on multiple lines;
- remove PREFER_DEFINED_ATTRIBUTE_MACRO, as this is kernel-specific;
- remove MACRO_ARG_REUSE, as this applies mostly to multithreading;
- don't warn on using do{} while(0) with single line statements;

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>