[PATCH] dmi: make dmi_string() behave like strdup()

This patch changes dmi_string() function to allocate string copy by itself, to
avoid code duplication in the next patch.

Signed-off-by: Andrey Panin <pazke@donpac.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>