reduce latency in __purge_vmap_area_lazy

From: Christoph Hellwig
Date: Sat Oct 22 2016 - 11:17:38 EST

Hi all,

this is my spin at sorting out the long lock hold times in
__purge_vmap_area_lazy. It is based on the patch from Joel sent this
week. I don't have any good numbers for it, but it survived an
xfstests run on XFS which is a significant vmalloc user.