file-posix: allow BLKZEROOUT with -t writeback

The Linux BLKZEROOUT ioctl is only invoked when BDRV_O_NOCACHE is set
because old kernels did not invalidate the page cache. In that case
mixing BLKZEROOUT with buffered I/O could lead to corruption.

However, Linux 4.9 commit 22dd6d356628 ("block: invalidate the page
cache when issuing BLKZEROOUT") made BLKZEROOUT coherent with the page
cache.

I have checked that Linux 4.9+ kernels are shipped at least as far back
as Debian 10 (buster), openSUSE Leap 15.2, and RHEL/CentOS 8.

Use BLKZEROOUT with buffered I/O, mostly so `qemu-img ... -t
writeback` can offload write zeroes.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250417211053.98700-1-stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/file-posix.c

@@ -785,17 +785,6 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     }
 #endif
 
-    if (S_ISBLK(st.st_mode)) {
-#ifdef __linux__
-        /* On Linux 3.10, BLKDISCARD leaves stale data in the page cache.  Do
-         * not rely on the contents of discarded blocks unless using O_DIRECT.
-         * Same for BLKZEROOUT.
-         */
-        if (!(bs->open_flags & BDRV_O_NOCACHE)) {
-            s->has_write_zeroes = false;
-        }
-#endif
-    }
 #ifdef __FreeBSD__
     if (S_ISCHR(st.st_mode)) {
         /*