af_unix: Fix inq_len update problem in partial read

af_unix: Fix inq_len update problem in partial read

Currently inq_len is updated only when the whole skb is consumed.
If only part of the data is read, following SIOCINQ query would
get value greater than what actually left.

This change update inq_len timely in unix_stream_read_generic(),
and adjust unix_stream_read_skb() accordingly to prevent
repetitive update.

Fixes: f4e1fb04c123 ("af_unix: Use cached value for SOCK_STREAM in unix_inq_len().")
Signed-off-by: Jianyu Li <jianyu.li@mediatek.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260601113640.231897-2-jianyu.li@mediatek.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>