mm: sync vmalloc address space page tables in alloc_vm_area()

mm: sync vmalloc address space page tables in alloc_vm_area()

Xen backend drivers (e.g., blkback and netback) would sometimes fail to
map grant pages into the vmalloc address space allocated with
alloc_vm_area().  The GNTTABOP_map_grant_ref would fail because Xen could
not find the page (in the L2 table) containing the PTEs it needed to
update.

(XEN) mm.c:3846:d0 Could not find L1 PTE for address fbb42000

netback and blkback were making the hypercall from a kernel thread where
task->active_mm != &init_mm and alloc_vm_area() was only updating the page
tables for init_mm.  The usual method of deferring the update to the page
tables of other processes (i.e., after taking a fault) doesn't work as a
fault cannot occur during the hypercall.

This would work on some systems depending on what else was using vmalloc.

Fix this by reverting ef691947d8a3 ("vmalloc: remove vmalloc_sync_all()
from alloc_vm_area()") and add a comment to explain why it's needed.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Keir Fraser <keir.xen@gmail.com>
Cc: <stable@kernel.org>		[3.0.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>