NFSD: Fix returned READDIR offset cookie

NFSD: Fix returned READDIR offset cookie

[ Upstream commit 0a8f37fb34a96267c656f7254e69bb9a2fc89fe4 ]

Code inspection shows that the server's NFSv3 READDIR implementation
handles offset cookies slightly differently than the NFSv2 READDIR,
NFSv3 READDIRPLUS, and NFSv4 READDIR implementations,
and there doesn't seem to be any need for this difference.

As a clean up, I copied the logic from nfsd3_proc_readdirplus().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>