dentry is always hashed and negative, inode - non-error, non-NULL and
non-directory.  In such conditions d_splice_alias() is equivalent to
"d_instantiate(dentry, inode) and return NULL", which simplifies the
downstream code and is consistent with the "have to create a new object"
case.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>