block/io: fallback to bounce buffer if BLKZEROOUT is not supported because of alignment

Commit 5634622bcb ("file-posix: allow BLKZEROOUT with -t writeback")
enables the BLKZEROOUT ioctl when using 'writeback' cache, regressing
certain 'qemu-img convert' invocations, because of a pre-existing
issue. Namely, the BLKZEROOUT ioctl might fail with errno EINVAL when
the request is shorter than the block size of the block device.
Fallback to the bounce buffer, similar to when the ioctl is not
supported at all, rather than treating such an error as fatal.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3257
Resolves: https://bugzilla.proxmox.com/show_bug.cgi?id=7197
Cc: qemu-stable@nongnu.org
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20260105143416.737482-1-f.ebner@proxmox.com>
[Added TODO comment describing a larger fix that could be implemented in
the future.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>