Linux 5.15 compat: get_acl()

Kernel commits

332f606b32b6 ovl: enable RCU'd ->get_acl()
0cad6246621b vfs: add rcu argument to ->get_acl() callback

Added compatibility code to detect the new ->get_acl() interface
and correctly handle the case where the new rcu argument is set.

Reviewed-by: Coleman Kane <ckane@colemankane.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12548