xfs: ensure dentry consistency when the orphanage adopts a file

When the orphanage adopts a file, that file becomes a child of the
orphanage.  The dentry cache may have entries for the orphanage
directory and the name we've chosen, so (1) make sure we abort if the
dcache has a positive entry because something's not right; and (2)
invalidate and purge negative dentries if the adoption goes through.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>