XArray: Fix xas_pause at ULONG_MAX

If we were unlucky enough to call xas_pause() when the index was at
ULONG_MAX (or a multi-slot entry which ends at ULONG_MAX), we would
wrap the index back around to 0 and restart the iteration from the
beginning.  Use the XAS_BOUNDS state to indicate that we should just
stop the iteration.

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