Unbreak the gcc build with sendfile_test after r343362

[ngie]

Summary

gcc 8.x is more pedantic than clang 7.x with format strings and the tests
passed void* variables while supplying %s (which is technically
incorrect).

Make the affected void* variables use char* storage instead to address
this issue, as the compiler will upcast the values to char*.

MFC after: 1 month
MFC with: r343362

Test Plan

make; sudo make install; kyua test -k /usr/tests/lib/libc/sys/Kyuafile sendfile_test

[asomers]

Approved. But I think the MFC reminder won't work unless you replace "MFC With" with "X-MFC-With" in your commit message.

[ngie]

In D18934#404418, @asomers wrote:

Approved. But I think the MFC reminder won't work unless you replace "MFC With" with "X-MFC-With" in your commit message.

It should work. The Committer's Guide was updated to use MFC with instead of X-MFC-With (the old form): https://www.freebsd.org/doc/en/articles/committers-guide/article.html#commit-log-message back almost 2 years ago by @emaste: https://svnweb.freebsd.org/doc?view=revision&revision=50227 ;).

[asomers]

In D18934#404425, @ngie wrote:

In D18934#404418, @asomers wrote:

Approved. But I think the MFC reminder won't work unless you replace "MFC With" with "X-MFC-With" in your commit message.

It should work. The Committer's Guide was updated to use MFC with instead of X-MFC-With (the old form): https://www.freebsd.org/doc/en/articles/committers-guide/article.html#commit-log-message back almost 2 years ago by @emaste: https://svnweb.freebsd.org/doc?view=revision&revision=50227 ;).

Oh, that's news to me. I guess I should reread the committer's guide every five years or so.