staging: lustre: fix error deref in ll_splice_alias().

d_splice_alias() can return an ERR_PTR().
If it does while debugging is enabled, the following
CDEBUG() will dereference that error and crash.

So add appropriate checking, and provide a separate
debug message for the error case.

Reported-and-tested-by: James Simmons <jsimmons@infradead.org>
Fixes: e9d4f0b9

 ("staging: lustre: llite: use d_splice_alias for directories.")
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>