libzfs: write_inuse_diffs_one: format strerror() with "%s"

Fixes 50353db ("Let zfs diff be more  permissive") which accidentally
introduced a build warning.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12197

lib/libzfs/libzfs_diff.c

@@ -262,7 +262,7 @@ write_inuse_diffs_one(FILE *fp, differ_info_t *di, uint64_t dobj)
 	fobjerr = get_stats_for_obj(di, di->fromsnap, dobj, fobjname,
 	    MAXPATHLEN, &fsb);
 	if (fobjerr && di->zerr != ENOTSUP && di->zerr != ENOENT) {
-		zfs_error_aux(di->zhp->zfs_hdl, strerror(di->zerr));
+		zfs_error_aux(di->zhp->zfs_hdl, "%s", strerror(di->zerr));
 		zfs_error(di->zhp->zfs_hdl, di->zerr, di->errbuf);
 		/*
 		 * Let's not print an error for the same object more than
@@ -276,7 +276,8 @@ write_inuse_diffs_one(FILE *fp, differ_info_t *di, uint64_t dobj)
 
 	if (tobjerr && di->zerr != ENOTSUP && di->zerr != ENOENT) {
 		if (!already_logged) {
-			zfs_error_aux(di->zhp->zfs_hdl, strerror(di->zerr));
+			zfs_error_aux(di->zhp->zfs_hdl,
+			    "%s", strerror(di->zerr));
 			zfs_error(di->zhp->zfs_hdl, di->zerr, di->errbuf);
 		}
 	}