Fix -Wformat-truncation warning in upgrade_set_callback()

Extend the buffer slightly resolve the warning.

    cmd/zfs/zfs_main.c: In function ‘upgrade_set_callback’:
    cmd/zfs/zfs_main.c:2446:22: error: ‘%llu’ directive output
    may be truncated writing between 1 and 20 bytes into a
    region of size 16 [-Werror=format-truncation=]
    cmd/zfs/zfs_main.c:2445:24: note: ‘snprintf’ output between
    2 and 21 bytes into a destination of size 16

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_main.c

@@ -2441,7 +2441,7 @@ upgrade_set_callback(zfs_handle_t *zhp, void *data)
 
 	/* upgrade */
 	if (version < cb->cb_version) {
-		char verstr[16];
+		char verstr[24];
 		(void) snprintf(verstr, sizeof (verstr),
 		    "%llu", (u_longlong_t)cb->cb_version);
 		if (cb->cb_lastfs[0] && !same_pool(zhp, cb->cb_lastfs)) {