mm: add preempt points into __purge_vmap_area_lazy()

mm: add preempt points into __purge_vmap_area_lazy()

Use cond_resched_lock to avoid holding the vmap_area_lock for a
potentially long time and thus creating bad latencies for various
workloads.

[hch: split from a larger patch by Joel, wrote the crappy changelog]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Joel Fernandes <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Tested-by: Jisheng Zhang <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: John Dias <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>