checkpatch: add warning for pr_* and dev_* macros without a trailing newline

checkpatch: add warning for pr_* and dev_* macros without a trailing newline

Add a new check in scripts/checkpatch.pl to detect usage of pr_(level) and
dev_(level) macros (for both C and Rust) when the string literal does not
end with '\n'.  Missing trailing newlines can lead to incomplete log lines
that do not appear properly in dmesg or in console output.  To show an
example of this working after applying the patch we can run the script on
the commit that likely motivated this need/issue:

  ./scripts/checkpatch.pl --strict -g "f431c5c581fa1"

Also, the patch is able to handle correctly if there is a printing call
without a newline which then has a newline printed via pr_cont for both
Rust and C alike.  If there is no newline printed and the patch ends or
there is another pr_* call before a newline with pr_cont is printed it
will show a warning.  Not implemented for dev_cont because it is not clear
to me if that is used at all.

One false warning that will be generated due to this change is in case we
have a patch that modifies a `pr_* call without a newline` which has a
pr_cont with a newline following it.  In this case there will be a warning
but because the patch does not include the following pr_cont it will warn
there is nothing creating a newline.  I have modified the warning to be
softer due to this known problem.

I have tested with comments, whitespace, differen orders of pr_* calls and
pr_cont and the only case that I suspect to be a problem is the one
outlined above.

Link: https://lkml.kernel.org/r/20250207-checkpatch-newline2-v4-1-26d8e80d0059@invicto.ai
Signed-off-by: Alban Kurti <kurti@invicto.ai>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://github.com/Rust-for-Linux/linux/issues/1140
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Joe Perches <joe@perches.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: Charalampos Mitrodimas <charmitro@posteo.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>