dcache: close d_move race in d_splice_alias

dcache: close d_move race in d_splice_alias

d_splice_alias will d_move an IS_ROOT() directory dentry into place if
one exists.  This should be safe as long as the dentry remains IS_ROOT,
but I can't see what guarantees that: once we drop the i_lock all we
hold here is the i_mutex on an unrelated parent directory.

Instead copy the logic of d_materialise_unique.

Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>
Signed-off-by: Al Viro <[email protected]>