nsfs: fix wrong error code returned for pidns ioctls

nsfs: fix wrong error code returned for pidns ioctls

When executing NS_GET_PID_FROM_PIDNS (or similar pidns ioctls), if the
target task cannot be found in the corresponding pid_ns, the error code
should be ESRCH instead of ENOTTY.

This bug was introduced when the extensible ioctl handling was added.
Without proper return, ret would be overwritten by the default case in
the extensible ioctl switch statement.

Fixes: a1d220d9dafa8 ("nsfs: iterate through mount namespaces")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://patch.msgid.link/20260507112301.1042757-1-chengzhihao1@huawei.com
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>