IDR: Expose the XArray lock

Allow users of the IDR to use the XArray lock for their own
synchronisation purposes.  The IDR continues to rely on the caller to
handle locking, but this lets the caller use the lock embedded in the
IDR data structure instead of allocating their own lock.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>