makefs/zfs: fix build with gcc 12 after b5a2bf512dbe
Due to integer promotion rules, dn_nlevels (uint8_t) gets promoted to a
plain int, resulting in -Werror warnings with gcc 12:
In file included from /workspace/src/usr.sbin/makefs/zfs.c:35:
/workspace/src/usr.sbin/makefs/zfs.c: In function '_dnode_cursor_flush':
/workspace/src/usr.sbin/makefs/zfs.c:684:23: error: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Werror=sign-compare]
684 | assert(levels <= c->dnode->dn_nlevels - 1);
| ^~
/workspace/src/usr.sbin/makefs/zfs.c:691:27: error: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Werror=sign-compare]
691 | if (level == c->dnode->dn_nlevels - 1) {
| ^~
/workspace/src/usr.sbin/makefs/zfs.c: In function 'dnode_cursor_next':
/workspace/src/usr.sbin/makefs/zfs.c:739:41: error: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Werror=sign-compare]
739 | for (levels = 0; levels < c->dnode->dn_nlevels - 1; levels++) {
| ^Fix it by subtracting 1U instead of 1.
Fixes: b5a2bf512dbe
MFC after: 3 days
(cherry picked from commit 8a77bc5e1be1a999f2f8d9df0c01fe01fb5a0787)