selftests/landlock: Add tests for access through disconnected paths

selftests/landlock: Add tests for access through disconnected paths

This adds tests for the edge case discussed in [1], with specific ones
for rename and link operations when the operands are through
disconnected paths, as that go through a separate code path in Landlock.

This has resulted in a warning, due to collect_domain_accesses() not
expecting to reach a different root from path->mnt:

  #  RUN           layout1_bind.path_disconnected ...
  #            OK  layout1_bind.path_disconnected
  ok 96 layout1_bind.path_disconnected
  #  RUN           layout1_bind.path_disconnected_rename ...
  [..] ------------[ cut here ]------------
  [..] WARNING: CPU: 3 PID: 385 at security/landlock/fs.c:1065 collect_domain_accesses
  [..] ...
  [..] RIP: 0010:collect_domain_accesses (security/landlock/fs.c:1065 (discriminator 2) security/landlock/fs.c:1031 (discriminator 2))
  [..] current_check_refer_path (security/landlock/fs.c:1205)
  [..] ...
  [..] hook_path_rename (security/landlock/fs.c:1526)
  [..] security_path_rename (security/security.c:2026 (discriminator 1))
  [..] do_renameat2 (fs/namei.c:5264)
  #            OK  layout1_bind.path_disconnected_rename
  ok 97 layout1_bind.path_disconnected_rename

Move the const char definitions a bit above so that we can use the path
for s4d1 in cleanup code.

Cc: Günther Noack <gnoack@google.com>
Cc: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/027d5190-b37a-40a8-84e9-4ccbc352bcdf@maowtm.org
Signed-off-by: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/20251128172200.760753-4-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>