NFSD: Change the way the expected length of a fattr4 is checked

NFSD: Change the way the expected length of a fattr4 is checked

[ Upstream commit 081d53fe0b43c47c36d1832b759bf14edde9cdbb ]

Because the fattr4 is now managed in an xdr_stream, all that is
needed is to store the initial position of the stream before
decoding the attribute list. Then the actual length of the list
is computed using the final stream position, after decoding is
complete.

No behavior change is expected.

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