qnx4: Extract dir entry filename processing into helper

Both dir.c and namei.c need to perform the same work to figure out a
directory entry's name and size. Extract this into a helper for use
in the next patch.

Acked-by: Anders Larsen <al@alarsen.net>
Link: https://lore.kernel.org/r/20231130205120.3642477-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>