zram: Replace bit spinlocks with spinlock_t for PREEMPT_RT.

The bit spinlock disables preemption on PREEMPT_RT. With disabled preemption it
is not allowed to acquire other sleeping locks which includes invoking
zs_free().

Use a spinlock_t on PREEMPT_RT for locking and set/ clear ZRAM_LOCK after the
lock has been acquired/ dropped.

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/YqIbMuHCPiQk+Ac2@linutronix.de