Fix problems in zvol_set_volmode_impl

- Don't leave fstrans set when passed a snapshot
- Don't remove minor if volmode already matches new value
- (FreeBSD) Wait for GEOM ops to complete before trying
  remove (at create time GEOM will be "tasting" in parallel)
- (FreeBSD) Don't leak zvol_state_lock on open if zv == NULL
- (FreeBSD) Don't try to unlock zv->zv_state lock if zv == NULL

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11199

module/os/linux/zfs/zvol_os.c

@@ -781,6 +781,7 @@ zvol_alloc(dev_t dev, const char *name)
 	zv = kmem_zalloc(sizeof (zvol_state_t), KM_SLEEP);
 	zso = kmem_zalloc(sizeof (struct zvol_state_os), KM_SLEEP);
 	zv->zv_zso = zso;
+	zv->zv_volmode = volmode;
 
 	list_link_init(&zv->zv_next);
 	mutex_init(&zv->zv_state_lock, NULL, MUTEX_DEFAULT, NULL);
@@ -886,6 +887,11 @@ zvol_free(zvol_state_t *zv)
 	kmem_free(zv, sizeof (zvol_state_t));
 }
 
+void
+zvol_wait_close(zvol_state_t *zv)
+{
+}
+
 /*
  * Create a block device minor node and setup the linkage between it
  * and the specified volume.  Once this function returns the block