x86/vdso/vdso2c: Correct error messages on file open

err() message in main() is misleading: it should print `outfilename`,
which is argv[3], not argv[2].

Correct error messages to be more precise about what failed and for
which file.

Co-developed-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200420183256.660371-2-dima@arista.com