arm64/spinlock: fix a -Wunused-function warning

arm64/spinlock: fix a -Wunused-function warning

The commit 47d441131e33 ("locking/osq: Use optimized spinning loop for
arm64") introduced a warning from Clang because vcpu_is_preempted() is
compiled away,

kernel/locking/osq_lock.c:25:19: warning: unused function 'node_cpu'
[-Wunused-function]
static inline int node_cpu(struct optimistic_spin_node *node)
                  ^
1 warning generated.

Fix it by converting vcpu_is_preempted() to a static inline function.

Fixes: 47d441131e33 ("locking/osq: Use optimized spinning loop for arm64")
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Will Deacon <will@kernel.org>