CVE-2025-21643: netfs: Fix kernel async DIO

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix kernel async DIO

Netfslib needs to be able to handle kernel-initiated asynchronous DIO that is supplied with a biovec[] array. Currently, because of the async flag, this gets passed to netfsextractuseriter() which throws a warning and fails because it only handles IOVEC and UBUF iterators. This can be triggered through a combination of cifs and a loopback blockdev with something like:

mount //my/cifs/share /foo dd if=/dev/zero of=/foo/m0 bs=4K count=1K losetup --sector-size 4096 --direct-io=on /dev/loop2046 /foo/m0 echo hello >/dev/loop2046

This causes the following to appear in syslog:

WARNING: CPU: 2 PID: 109 at fs/netfs/iterator.c:50 netfsextractuseriter+0x170/0x250 [netfs]

and the write to fail.

Fix this by removing the check in netfsunbufferedwriteiterlocked() that causes async kernel DIO writes to be handled as userspace writes. Note that this change relies on the kernel caller maintaining the existence of the biovec array (or kvec[] or folioqueue) until the op is complete.