maple tree: Store plain node in ms->node

maple tree: Store plain node in ms->node

Abandon the _maple_mk_node encoding in the ms->node (a future patch will
use this bit pattern for encoding errors like the XArray).  We also need
to distinguish between "Start walking" and "We have found the entry;
it's in the root".  Also correct a few xa_is_internal() that should have
been xa_is_node().

Signed-off-by: Matthew Wilcox <willy@infradead.org>