FreeBSD: Fix a potential null dereference in zfs_freebsd_fsync()

In general it's possible for a vnode to not have an associated VM
object.  This happens in particular with named pipes, which have
some distinct VOPs, defined in zfs_fifoops.  Thus, this chunk of
zfs_freebsd_fsync() needs to check for the FIFO case, like other
vm_object_mightbedirty() callers do.

(Note that vn_flush_cached_data() calls are predicated on
zn_has_cached_data() returning true, and it checks for a NULL v_object
pointer already.)

Fixes: ef4058fcdc
Reported-by: Collin Funk <[email protected]>
Reviewed-by: Sean Eric Fagan <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Mark Johnston <[email protected]>
Closes #18015

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -5277,7 +5277,7 @@ zfs_freebsd_fsync(struct vop_fsync_args *ap)
 	 * Push any dirty mmap()'d data out to the DMU and ZIL, ready for
 	 * zil_commit() to be called in zfs_fsync().
 	 */
-	if (vm_object_mightbedirty(vp->v_object)) {
+	if (vp->v_object != NULL && vm_object_mightbedirty(vp->v_object)) {
 		zfs_vmobject_wlock(vp->v_object);
 		if (!vm_object_page_clean(vp->v_object, 0, 0, 0))
 			err = SET_ERROR(EIO);