ARM: vfp: Use vfp_lock() in vfp_entry().

vfp_entry() is invoked from exception handler and is fully preemptible.
It uses local_bh_disable() to remain uninterrupted while checking the
VFP state.
This is not working on PREEMPT_RT because local_bh_disable()
synchronizes the relevant section but the context remains fully
preemptible.

Use vfp_lock() for uninterrupted access.

VFP_bounce() is invoked from within vfp_entry() and may send a signal.
Sending a signal uses spinlock_t which becomes a sleeping lock
on PREEMPT_RT and must not be acquired within a preempt-disabled
section. Move the vfp_raise_sigfpe() block outside of the
preempt-disabled section.

Link: https://lore.kernel.org/r/20230519145731.574867-4-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>