XArray: Redesign xa_alloc API

It was too easy to forget to initialise the start index.  Add an
xa_limit data structure which can be used to pass min & max, and
define a couple of special values for common cases.  Also add some
more tests cribbed from the IDR test suite.  Change the return value
from -ENOSPC to -EBUSY to match xa_insert().

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