dax: Reinstate RCU protection of inode

For the device-dax case, it is possible that the inode can go away
underneath us.  The rcu_read_lock() was there to prevent it from
being freed, and not (as I thought) to protect the tree.  Bring back
the rcu_read_lock() protection.  Also add a little kernel-doc; while
this function is not exported to modules, it is used from outside dax.c

Reported-by: Dan Williams <dan.j.williams@intel.com>
Fixes: 9f32d221

 ("dax: Convert dax_lock_mapping_entry to XArray")
Signed-off-by: Matthew Wilcox <willy@infradead.org>