Specifying a starting ID greater than the maximum ID isn't something
attempted very often, but it should fail. It was succeeding due to
xas_find_marked() returning the wrong error state, so add tests for
both xa_alloc() and xas_find_marked().
Fixes: b803b42823 ("xarray: Add XArray iterators")
Signed-off-by: Matthew Wilcox <willy@infradead.org>