mm: turn vmap_purge_lock into a mutex

The purge_lock spinlock causes high latencies with non RT kernel.  This
has been reported multiple times on lkml [1] [2] and affects
applications like audio.

This patch replaces it with a mutex to allow preemption while holding
the lock.

Thanks to Joel Fernandes for the detailed report and analysis as well as
an earlier attempt at fixing this issue.

[1] http://lists.openwall.net/linux-kernel/2016/03/23/29
[2] https://lkml.org/lkml/2016/10/9/59

Link: http://lkml.kernel.org/r/1479474236-4139-10-git-send-email-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Jisheng Zhang <jszhang@marvell.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: John Dias <joaodias@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>