NFS: Fix handling of cookie verifier in uncached_readdir()

NFS: Fix handling of cookie verifier in uncached_readdir()

[ Upstream commit 13884ff2bef01df37c450c6dd09122f92333dccc ]

If we're doing uncached readdir(), then the readdir cookie could be
different from the one cached in the nfs_inode. We should therefore
ensure that we save that one in the struct nfs_open_dir_context.

Fixes: 35df59d3ef69 ("NFS: Reduce number of RPC calls when doing uncached readdir")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>