lib/generic-radix-tree.c: Make nodes more reasonably sized

this code originally used the page allocator directly, but most code
shouldn't do that - PAGE_SIZE varies with architecture, and slab is
faster.

4k is also on the large side for typical usage, 512 bytes is a better
choice for typical usage that might be somewhat sparse.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>