Address warnings about possible division by zero from clangsa

* The complaint in ztest_replay_write() is only possible if something
   went horribly wrong. An assertion will silence this and if it goes
   off, we will know that something is wrong.
 * The complaint in spa_estimate_metaslabs_to_flush() is not impossible,
   but seems very unlikely. We resolve this by passing the value from
   the `MIN()` that does not go to infinity when the variable is zero.

There was a third report from Clang's scan-build, but that was a
definite false positive and disappeared when checked again through
Clang's static analyzer with Z3 refution via CodeChecker.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14124

cmd/ztest.c

@@ -2177,6 +2177,7 @@ ztest_replay_write(void *arg1, void *arg2, boolean_t byteswap)
 		 * but not always, because we also want to verify correct
 		 * behavior when the data was not recently read into cache.
 		 */
+		ASSERT(doi.doi_data_block_size);
 		ASSERT0(offset % doi.doi_data_block_size);
 		if (ztest_random(4) != 0) {
 			int prefetch = ztest_random(2) ?

module/zfs/spa_log_spacemap.c

@@ -691,7 +691,8 @@ spa_estimate_metaslabs_to_flush(spa_t *spa)
 		 * based on the incoming rate until we exceed it.
 		 */
 		if (available_blocks >= 0 && available_txgs >= 0) {
-			uint64_t skip_txgs = MIN(available_txgs + 1,
+			uint64_t skip_txgs = (incoming == 0) ?
+			    available_txgs + 1 : MIN(available_txgs + 1,
 			    (available_blocks / incoming) + 1);
 			available_blocks -= (skip_txgs * incoming);
 			available_txgs -= skip_txgs;