[PATCH] radix_tag_get(): differentiate between no present node and tag unset cases

Simple patch to radix_tree_tag_get() to return different values for non
present node and tag unset.

The function is not used by any in-kernel callers (yet), but this
information is definitely useful.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>