dcache: move d_splice_alias

Just a trivial move to locate it near (similar) d_materialise_unique
code and save some forward references in a following patch.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>