UBUNTU: SAUCE: mm: disable vma based swap readahead by default

UBUNTU: SAUCE: mm: disable vma based swap readahead by default

BugLink: http://bugs.launchpad.net/bugs/1732463
Starting with 4.14 our test for CVE-2015-7550 started oopsing the
kernel on i386 with the following stack trace:

  EIP: __radix_tree_lookup+0x15/0xf0
  EFLAGS: 00010292 CPU: 2
  EAX: 00000004 EBX: b7e77000 ECX: 00000000 EDX: 00000000
  ESI: 00000000 EDI: 00000000 EBP: dcfb1da4 ESP: dcfb1d8c
   DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
  CR0: 80050033 CR2: 00000008 CR3: 1cd15dc0 CR4: 000006f0
  Call Trace:
   radix_tree_lookup_slot+0x1d/0x40
   find_get_entry+0x22/0xe0
   pagecache_get_page+0x24/0x220
   lookup_swap_cache+0x3a/0xf0
   swap_readahead_detect+0x55/0x280
   do_swap_page+0x232/0x890
   ? keyring_search_aux+0x96/0xb0
   ? kmap_atomic_prot+0xe0/0x100
   handle_mm_fault+0x6f8/0xe20
   ? lookup_user_key+0xef/0x450
   __do_page_fault+0x22d/0x540
   ? kvm_async_pf_task_wake+0x100/0x100
   do_page_fault+0x27/0xe0
   ? kvm_async_pf_task_wake+0x100/0x100
   do_async_page_fault+0x55/0x90
   common_exception+0x6f/0x76

I'm not able to reproduce this outside of ADT, but vma based swap
readahead is a new feature in 4.14 so it seems quite likely that
this is where the bug lies. However I'm not able to reproduce the
problem outside of ADT to confirm this.

So for now disable this feature by default so we can see if that
gets the test to pass. It can still be enabled by writing to
/sys/kernel/mm/swap/vma_ra_enabled if desired.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>