Reimplement rt_mutex_owner to fix build with DEBUG & PREEMPT_RT_FULL

Reimplement rt_mutex_owner to fix build with DEBUG & PREEMPT_RT_FULL

rt_mutex_owner is internal to kernel/locking/rtmutex_common.h and
inaccessible for SPL via the public kernel headers. The way of
accessing the owner has been stable since at least 3.13 ([1], [2]),
which is masking the lowest bit in the owner pointer in rt_mutex. We
do the same.

[1] http://lxr.free-electrons.com/source/kernel/locking/rtmutex_common.h?v=3.13#L99
[2] http://lxr.free-electrons.com/source/kernel/locking/rtmutex_common.h?v=4.9#L78

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>
Closes #593