NFS: Return valid errors from nfs2/3_decode_dirent()

[ Upstream commit 64cfca85bacde54caa64e0ab855c48734894fa37 ]

Valid return values for decode_dirent() callback functions are:
 0: Success
 -EBADCOOKIE: End of directory
 -EAGAIN: End of xdr_stream

All errors need to map into one of those three values.

Fixes: 573c4e1e ("NFS: Simplify ->decode_dirent() calling sequence")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>