scripts/faddr2line: Check vmlinux only once

Rather than checking whether the object file is vmlinux for each invocation
of __faddr2line, check it only once beforehand.

Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Link: https://lore.kernel.org/r/20240415145538.1938745-4-bjohannesmeyer@gmail.com


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>