spinlock: Fix failing build for PREEMPT_RT

Since 1d71b30e1f85 ("sched.h: Move (spin|rwlock)_needbreak() to
spinlock.h") build fails for PREEMPT_RT, since there is no definition
available of either spin_needbreak() and rwlock_needbreak().

Since it was moved on the mentioned commit, it was placed inside a
!PREEMPT_RT part of the code, making it out of reach for an RT kernel.

Fix this by moving code it a few lines down so it can be reached by an
RT build, where it can also make use of the *_is_contended() definition
added by the spinlock_rt.h.

Fixes: d1d71b30 ("sched.h: Move (spin|rwlock)_needbreak() to
spinlock.h")
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Acked-by: Waiman Long <longman@redhat.com>