mm: kmemleak: make the tool tolerant to struct scan_area allocation failures

Patch series "mm: kmemleak: Use a memory pool for kmemleak object
allocations", v3.

Following the discussions on v2 of this patch(set) [1], this series takes
slightly different approach:

- it implements its own simple memory pool that does not rely on the
  slab allocator

- drops the early log buffer logic entirely since it can now allocate
  metadata from the memory pool directly before kmemleak is fully
  initialised

- CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option is renamed to
  CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE

- moves the kmemleak_init() call earlier (mm_init())

- to avoid a separate memory pool for struct scan_area, it makes the
  tool robust when such allocations fail as scan areas are rather an
  optimisation

[1] http://lkml.kernel.org/r/20190727132334.9184-1-catalin.marinas@arm.com

This patch (of 3):

Object scan areas are an optimisation aimed to decrease the false
positives and slightly improve the scanning time of large objects known to
only have a few specific pointers.  If a struct scan_area fails to
allocate, kmemleak can still function normally by scanning the full
object.

Introduce an OBJECT_FULL_SCAN flag and mark objects as such when scan_area
allocation fails.

Link: http://lkml.kernel.org/r/20190812160642.52134-2-catalin.marinas@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>