NFS: always treat the invocation of nfs_getattr as cache hit when noac is on

When using 'ls -l' to display a large directory, if noac option is used,
in function nfs_getattr() nfs_need_revalidate_inode() will always be true
for NFSv3 and the nfs_entry cache of the directory will be flushed. The
flush will lead to a fully reread of the directory entries from server.

To prevent the unnecessary RPCs, we need to check whether or not the
noac option is used, and always report the invocation of nfs_getattr()
as cache hit instead cache miss when it's on.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>