XArray: Fix xa_alloc when id exceeds max

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: b803b428

 ("xarray: Add XArray iterators")
Signed-off-by: Matthew Wilcox <willy@infradead.org>