net: nfs: Fix CVE-2022-30767 (old CVE-2019-14196)

This patch mitigates the vulnerability identified via CVE-2019-14196.

The previous patch was bypassed/ineffective, and now the vulnerability
is identified via CVE-2022-30767. The patch removes the sanity check
introduced to mitigate CVE-2019-14196 since it's ineffective.
filefh3_length is changed to unsigned type integer, preventing negative
numbers from being used during comparison with positive values during
size sanity checks.

Signed-off-by: Andrea zi0Black Cappa <zi0Black@protonmail.com>