9p/net: return error on bogus (longer than requested) replies

Up until now we've been considering longer than requested replies as
acceptable, printing a message and just truncating the data,
but it makes more sense to consider these an error.

Make these fail with EIO instead.

Suggested-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Message-ID: <20250317-p9_bogus_io_error-v1-1-9639f6d1561f@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>