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: ef4058fcdc01838117dd93a654228bac7487a37c
Reported-by: Collin Funk <collin.funk1@gmail.com>
Reviewed-by: Sean Eric Fagan <sef@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #18015