landlock: Fix warning from KUnit tests

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

Fixes: d9d2a68ed44b ("landlock: Add unique ID generator")
Signed-off-by: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/73e28efc5b8cc394608b99d5bc2596ca917d7c4a.1750003733.git.m@maowtm.org
[mic: Minor cosmetic improvements]
Signed-off-by: Mickaël Salaün <mic@digikod.net>