aufs: i_op: Add handling for au_pin_hdir_set_owner with RT kernel

In RT kernel rw_semaphore uses rt_mutex whose owner should be set to the
task. Add a condition to handle both cases to avoid the following build error
for PREEMPT_RT kernel.

fs/aufs/i_op.c: In function 'au_pin_hdir_set_owner':
fs/aufs/i_op.c:639:52: error: 'struct rw_semaphore' has no member named 'owner'
639 |         atomic_long_set(&p->hdir->hi_inode->i_rwsem.owner, (long)task);

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>