pNFS: Prevent unnecessary layoutreturns after delegreturn

If we cannot grab the inode or superblock, then we cannot pin the
layout header, and so we cannot send a layoutreturn as part of an
async delegreturn call. In this case, we currently end up sending
an extra layoutreturn after the delegreturn. Since the layout was
implicitly returned by the delegreturn, that just gets a BAD_STATEID.

The fix is to simply complete the return-on-close immediately.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>