ABD style cleanups

The commit a6255b7 removed a few
assertions which help catch errors and improve code readability. It also
duplicated two conditionals, which was unnecessary and made the code
confusing to read. This patch cleans it up.

Reviewed-by: David Quigley <david.quigley@intel.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes openzfs#5802

module/zfs/vdev_raidz.c

@@ -591,20 +591,20 @@ vdev_raidz_generate_parity_pq(raidz_map_t *rm)
 		ccnt = rm->rm_col[c].rc_size / sizeof (p[0]);
 
 		if (c == rm->rm_firstdatacol) {
+			ASSERT(ccnt == pcnt || ccnt == 0);
 			abd_copy_to_buf(p, src, rm->rm_col[c].rc_size);
 			(void) memcpy(q, p, rm->rm_col[c].rc_size);
-		} else {
-			struct pqr_struct pqr = { p, q, NULL };
-			(void) abd_iterate_func(src, 0, rm->rm_col[c].rc_size,
-			    vdev_raidz_pq_func, &pqr);
-		}
 
-		if (c == rm->rm_firstdatacol) {
 			for (i = ccnt; i < pcnt; i++) {
 				p[i] = 0;
 				q[i] = 0;
 			}
 		} else {
+			struct pqr_struct pqr = { p, q, NULL };
+
+			ASSERT(ccnt <= pcnt);
+			(void) abd_iterate_func(src, 0, rm->rm_col[c].rc_size,
+			    vdev_raidz_pq_func, &pqr);
 
 			/*
 			 * Treat short columns as though they are full of 0s.
@@ -639,22 +639,23 @@ vdev_raidz_generate_parity_pqr(raidz_map_t *rm)
 		ccnt = rm->rm_col[c].rc_size / sizeof (p[0]);
 
 		if (c == rm->rm_firstdatacol) {
+			ASSERT(ccnt == pcnt || ccnt == 0);
 			abd_copy_to_buf(p, src, rm->rm_col[c].rc_size);
 			(void) memcpy(q, p, rm->rm_col[c].rc_size);
 			(void) memcpy(r, p, rm->rm_col[c].rc_size);
-		} else {
-			struct pqr_struct pqr = { p, q, r };
-			(void) abd_iterate_func(src, 0, rm->rm_col[c].rc_size,
-			    vdev_raidz_pqr_func, &pqr);
-		}
 
-		if (c == rm->rm_firstdatacol) {
 			for (i = ccnt; i < pcnt; i++) {
 				p[i] = 0;
 				q[i] = 0;
 				r[i] = 0;
 			}
 		} else {
+			struct pqr_struct pqr = { p, q, r };
+
+			ASSERT(ccnt <= pcnt);
+			(void) abd_iterate_func(src, 0, rm->rm_col[c].rc_size,
+			    vdev_raidz_pqr_func, &pqr);
+
 			/*
 			 * Treat short columns as though they are full of 0s.
 			 * Note that there's therefore nothing needed for P.