NFS/pNFS: Fix a leak of the layout 'plh_outstanding' counter

If we exit _lgopen_prepare_attached() without setting a layout, we will
currently leak the plh_outstanding counter.

Fixes: 411ae722 ("pNFS: Wait for stale layoutget calls to complete in pnfs_update_layout()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>