selftests/landlock: Add 11 new test suites dedicated to file reparenting

These test suites try to check all edge cases for directory and file
renaming or linking involving a new parent directory, with and without
LANDLOCK_ACCESS_FS_REFER and other access rights.

layout1:
* reparent_refer: Tests simple FS_REFER usage.
* reparent_link: Tests a mix of FS_MAKE_REG and FS_REFER with links.
* reparent_rename: Tests a mix of FS_MAKE_REG and FS_REFER with renames
  and RENAME_EXCHANGE.
* reparent_exdev_layers_rename1/2: Tests renames with two layers.
* reparent_exdev_layers_exchange1/2/3: Tests exchanges with two layers.
* reparent_remove: Tests file and directory removal with rename.
* reparent_dom_superset: Tests access partial ordering.

layout1_bind:
* reparent_cross_mount: Tests FS_REFER propagation across mount points.

Test coverage for security/landlock is 95.4% of 604 lines according to
gcc/gcov-11.

Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-9-mic@digikod.net