landlock: Fix warning from KUnit tests

In the Linux kernel, the following vulnerability has been resolved:

landlock: Fix warning from KUnit tests

get_id_range() expects a positive value as first argument but
get_random_u8() can return 0. Fix this by clamping it.

Validated by running the test in a for loop for 1000 times.

Note that MAX() is wrong as it is only supposed to be used for
constants, but max() is good here.

[..] ok 9 test_range2_rand1
[..] ok 10 test_range2_rand2
[..] ok 11 test_range2_rand15
[..] ------------[ cut here ]------------
[..] WARNING: CPU: 6 PID: 104 at security/landlock/id.c:99 test_range2_rand16 (security/landlock/id.c:99 (discriminator 1) security/landlock/id.c:234 (discriminator 1))
[..] Modules linked in:
[..] CPU: 6 UID: 0 PID: 104 Comm: kunit_try_catch Tainted: G N 6.16.0-rc1-dev-00001-g314a2f98b65f #1 PREEMPT(undef)
[..] Tainted: [N]=TEST
[..] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[..] RIP: 0010:test_range2_rand16 (security/landlock/id.c:99 (discriminator 1) security/landlock/id.c:234 (discriminator 1))
[..] Code: 49 c7 c0 10 70 30 82 4c 89 ff 48 c7 c6 a0 63 1e 83 49 c7 45 a0 e0 63 1e 83 e8 3f 95 17 00 e9 1f ff ff ff 0f 0b e9 df fd ff ff <0f> 0b ba 01 00 00 00 e9 68 fe ff ff 49 89 45 a8 49 8d 4d a0 45 31

[..] RSP: 0000:ffff888104eb7c78 EFLAGS: 00010246
[..] RAX: 0000000000000000 RBX: 000000000870822c RCX: 0000000000000000
^^^^^^^^^^^^^^^^
[..]
[..] Call Trace:
[..]
[..] ---[ end trace 0000000000000000 ]---
[..] ok 12 test_range2_rand16
[..] # landlock_id: pass:12 fail:0 skip:0 total:12
[..] # Totals: pass:12 fail:0 skip:0 total:12
[..] ok 1 landlock_id

[mic: Minor cosmetic improvements]