NFSv4: Simplify some cache consistency post-op GETATTRs

Certain asynchronous operations such as write() do not expect
(or care) that other metadata such as the file owner, mode, acls, ...
change. All they want to do is update and/or check the change attribute,
ctime, and mtime.
By skipping the file owner and group update, we also avoid having to do a
potential idmapper upcall for these asynchronous RPC calls.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>