XArray: Document the locking requirement for the xa_state

It wasn't obvious to all readers that it's unsafe to reuse an xa_state
after dropping the xas_lock() or the rcu_read_lock().

Reported-by: Charan Teja Kalla <charante@codeaurora.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>