selftests/fuse: add ACL_DONT_CACHE regression test

selftests/fuse: add ACL_DONT_CACHE regression test

Add a test that reproduces the stale ACL bug fixed by:
  "fs: preserve ACL_DONT_CACHE state in forget_cached_acl()"

A FUSE mount that does not negotiate FUSE_POSIX_ACL initialises inodes
with i_acl = ACL_DONT_CACHE.  Before the fix, calling
forget_all_cached_acls() (e.g. from fuse_update_get_attr() on a
statx(AT_STATX_FORCE_SYNC)) would silently replace ACL_DONT_CACHE with
ACL_NOT_CACHED, enabling the kernel ACL cache.  A subsequent getxattr
would populate the cache, and because fuse_set_acl() skips
forget_all_cached_acls() for !fc->posix_acl, later ACL changes were
not visible to callers — getxattr returned stale data.

The test mounts a minimal libfuse3 lowlevel filesystem (no
FUSE_POSIX_ACL negotiated) and:
  1. Issues two getxattrs — both must reach the daemon, proving
     ACL_DONT_CACHE suppresses caching before any trigger.
  2. Calls statx(AT_STATX_FORCE_SYNC) to trigger forget_all_cached_acls().
  3. Issues another getxattr (populates the cache on a buggy kernel).
  4. Switches the daemon to a different-sized ACL (ACL_B).
  5. Issues a final getxattr — expects ACL_B (44 bytes) and daemon
     call count 4; a buggy kernel returns stale ACL_A (28 bytes).

fuse_acl_cache_test is only built when libfuse3 is detected via
pkg-config.

Christian Brauner <brauner@kernel.org> says:
Changed do_force_statx() to call the statx() libc wrapper instead of
syscall(SYS_statx, ...) as requested by Amir after review feedback from
Luis Henriques, and dropped the now unused <sys/syscall.h> include.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20260713220932.413004-3-amir73il@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>