lib/generic-radix-tree.c: Don't overflow in peek()

[ Upstream commit 9492261f ]

When we started spreading new inode numbers throughout most of the 64
bit inode space, that triggered some corner case bugs, in particular
some integer overflows related to the radix tree code. Oops.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>