NFSv4.1: Don't trust attributes if a pNFS LAYOUTCOMMIT is outstanding

If a LAYOUTCOMMIT is outstanding, then chances are that the metadata
server may still be returning incorrect values for the change attribute,
ctime, mtime and/or size.
Just ignore those attributes for now, and wait for the LAYOUTCOMMIT
rpc call to finish.

Reported-by: shaobingqing <shaobingqing@bwstor.com.cn>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>