staging: lustre: llite: use d_splice_alias for directories.

In the Linux dcache a directory only ever has one dentry,
so d_splice_alias() can be used by ll_splice_alias() for directories.
It will find the one dentry whether it is DCACHE_DISCONNECTED or
IS_ROOT() or d_lustre_invalid().
Separating out the directories from non-directories will allow us
to simplify the non-directory code.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>