include/linux/vmpressure.h: use spinlock_t instead of struct spinlock

include/linux/vmpressure.h: use spinlock_t instead of struct spinlock

For spinlocks the type spinlock_t should be used instead of "struct
spinlock".

Use spinlock_t for spinlock's definition.

Link: http://lkml.kernel.org/r/20190704153803.12739-3-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>