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

Fixes 50353dbd ("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