Cleanup: metaslab_alloc_dva() should not NULL check mg->mg_next

This is a circularly linked list. mg->mg_next can never be NULL.

This caused 3 defect reports in Coverity.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14042

Author: ryao

module/zfs/metaslab.c

@@ -5131,8 +5131,7 @@ metaslab_alloc_dva(spa_t *spa, metaslab_class_t *mc, uint64_t psize,
 		if (vd != NULL && vd->vdev_mg != NULL) {
 			mg = vdev_get_mg(vd, mc);
 
-			if (flags & METASLAB_HINTBP_AVOID &&
-			    mg->mg_next != NULL)
+			if (flags & METASLAB_HINTBP_AVOID)
 				mg = mg->mg_next;
 		} else {
 			mg = mca->mca_rotor;