kheaders: prevent `find` from seeing perl temp files

Symptom:

The command

    find ... | xargs ... perl -i

occasionally triggers error messages like the following, with the build
still succeeding:

    Can't open <redacted>/kernel/.tmp_dir/include/dt-bindings/clock/XXNX4nW9: No such file or directory.

Analysis:

With strace, the root cause has been identified to be `perl -i` creating
temporary files inside ${tmpdir}, which causes `find` to see the
temporary files and emit the names. `find` is likely implemented with
readdir. POSIX `readdir` says:

    If a file is removed from or added to the directory after the most
    recent call to opendir() or rewinddir(), whether a subsequent call
    to readdir() returns an entry for that file is unspecified.

So if the libc that `find` links against choose to return that entry
in readdir(), a possible sequence of events is the following:

1. find emits foo.h
2. xargs executes `perl -i foo.h`
3. perl (pid=100) creates temporary file `XXXXXXXX`
4. find sees file `XXXXXXXX` and emit it
5. PID 100 exits, cleaning up the temporary file `XXXXXXXX`
6. xargs executes `perl -i XXXXXXXX`
7. perl (pid=200) tries to read the file, but it doesn't exist any more.

... triggering the error message.

One can reproduce the bug with the following command (assuming PWD
contains the list of headers in kheaders.tar.xz)

    for i in $(seq 100); do
        find -type f -print0 |
            xargs -0 -P8 -n1 perl -pi -e 'BEGIN {undef $/;}; s/\/\*((?!SPDX).)*?\*\///smg;';
    done

With a `find` linking against musl libc, the error message is emitted
6/100 times.

The fix:

This change stores the results of `find` before feeding them into xargs.
find and xargs will no longer be able to see temporary files that perl
creates after this change.

Signed-off-by: HONG Yifan <elsk@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>