Fix -Wformat-overflow warning in zfs_project_handle_dir()

Switch to using asprintf() to satisfy the compiler and resolve the
potential format-overflow warning.  Not the conditional before the
sprintf() would have prevented this regardless.

    cmd/zfs/zfs_project.c: In function ‘zfs_project_handle_dir’:
    cmd/zfs/zfs_project.c:241:38: error: ‘/’ directive writing
    1 byte into a region of size between 0 and 4352
    [-Werror=format-overflow=]
    cmd/zfs/zfs_project.c:241:17: note: ‘sprintf’ output between
    2 and 4609 bytes into a destination of size 4352

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#13528
Closes openzfs#13575

cmd/zfs/zfs_project.c

@@ -207,7 +207,6 @@ static int
 zfs_project_handle_dir(const char *name, zfs_project_control_t *zpc,
     list_t *head)
 {
-	char fullname[PATH_MAX];
 	struct dirent *ent;
 	DIR *dir;
 	int ret = 0;
@@ -227,21 +226,28 @@ zfs_project_handle_dir(const char *name, zfs_project_control_t *zpc,
 	zpc->zpc_ignore_noent = B_TRUE;
 	errno = 0;
 	while (!ret && (ent = readdir(dir)) != NULL) {
+		char *fullname;
+
 		/* skip "." and ".." */
 		if (strcmp(ent->d_name, ".") == 0 ||
 		    strcmp(ent->d_name, "..") == 0)
 			continue;
 
-		if (strlen(ent->d_name) + strlen(name) >=
-		    sizeof (fullname) + 1) {
+		if (strlen(ent->d_name) + strlen(name) + 1 >= PATH_MAX) {
 			errno = ENAMETOOLONG;
 			break;
 		}
 
-		sprintf(fullname, "%s/%s", name, ent->d_name);
+		if (asprintf(&fullname, "%s/%s", name, ent->d_name) == -1) {
+			errno = ENOMEM;
+			break;
+		}
+
 		ret = zfs_project_handle_one(fullname, zpc);
 		if (!ret && zpc->zpc_recursive && ent->d_type == DT_DIR)
 			zfs_project_item_alloc(head, fullname);
+
+		free(fullname);
 	}
 
 	if (errno && !ret) {