Call slowpath __slab_alloc() from within the bulk loop, as the side-effect
of this call likely repopulates c->freelist.

Choose to reenable local IRQs while calling slowpath.

Saving some optimizations for later.  E.g.  it is possible to extract
parts of __slab_alloc() and avoid the unnecessary and expensive (37
cycles) local_irq_{save,restore}.  For now, be happy calling
__slab_alloc() this lower icache impact of this func and I don't have to
worry about correctness.

Measurements on CPU CPU i7-4790K @ 4.00GHz
Baseline normal fastpath (alloc+free cost): 42 cycles(tsc) 10.601 ns

Bulk- fallback                   - this-patch
  1 -  58 cycles(tsc) 14.516 ns  -  49 cycles(tsc) 12.459 ns  improved 15.5%
  2 -  51 cycles(tsc) 12.930 ns  -  38 cycles(tsc)  9.605 ns  improved 25.5%
  3 -  49 cycles(tsc) 12.274 ns  -  34 cycles(tsc)  8.525 ns  improved 30.6%
  4 -  48 cycles(tsc) 12.058 ns  -  32 cycles(tsc)  8.036 ns  improved 33.3%
  8 -  46 cycles(tsc) 11.609 ns  -  31 cycles(tsc)  7.756 ns  improved 32.6%
 16 -  45 cycles(tsc) 11.451 ns  -  32 cycles(tsc)  8.148 ns  improved 28.9%
 30 -  79 cycles(tsc) 19.865 ns  -  68 cycles(tsc) 17.164 ns  improved 13.9%
 32 -  76 cycles(tsc) 19.212 ns  -  66 cycles(tsc) 16.584 ns  improved 13.2%
 34 -  74 cycles(tsc) 18.600 ns  -  63 cycles(tsc) 15.954 ns  improved 14.9%
 48 -  88 cycles(tsc) 22.092 ns  -  77 cycles(tsc) 19.373 ns  improved 12.5%
 64 -  80 cycles(tsc) 20.043 ns  -  68 cycles(tsc) 17.188 ns  improved 15.0%
128 -  99 cycles(tsc) 24.818 ns  -  89 cycles(tsc) 22.404 ns  improved 10.1%
158 -  99 cycles(tsc) 24.977 ns  -  92 cycles(tsc) 23.089 ns  improved  7.1%
250 - 106 cycles(tsc) 26.552 ns  -  99 cycles(tsc) 24.785 ns  improved  6.6%

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>