NFSv4: Don't discard the attributes returned by asynchronous DELEGRETURN

DELEGRETURN will always carry a reference to the inode except when
the latter is being freed, so let's ensure that we always use that
inode information to ensure close-to-open cache consistency, even
when the DELEGRETURN call is asynchronous.

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