NFSv4: Don't modify the change attribute cached in the inode

When the client is caching data and a write delegation is held, then the
server may send a CB_GETATTR to query the attributes. When this happens,
the client is supposed to bump the change attribute value that it
returns if it holds cached data.
However that process uses a value that is stored in the delegation. We
do not want to bump the change attribute held in the inode.

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