ARM: vfp: Provide vfp_lock() for VFP locking.

kernel_neon_begin() uses local_bh_disable() to ensure exclusive access
to the VFP unit. This is broken on PREEMPT_RT because a BH disabled
section remains preemptible on PREEMPT_RT.

Introduce vfp_lock() which uses local_bh_disable() and preempt_disable()
on PREEMPT_RT. Since softirqs are processed always in thread context,
disabling preemption is enough to ensure that the current context won't
get interrupted by something that is using the VFP. Use it in
kernel_neon_begin().

Link: https://lore.kernel.org/r/20230519145731.574867-2-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>