[PATCH] lib/string.c cleanup: remove pointless explicit casts

The first two hunks of the patch really belongs in patch 1, but I missed
them on the first pass and instead of redoing all 3 patches I stuck them in
this one.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>