FROMLIST: kfence: move the size check to the beginning of __kfence_alloc()

Check the allocation size before toggling kfence_allocation_gate.
This way allocations that can't be served by KFENCE will not result in
waiting for another CONFIG_KFENCE_SAMPLE_INTERVAL without allocating
anything.

Suggested-by: Marco Elver <elver@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org # 5.12+
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Marco Elver <elver@google.com>

Bug: 192294212
Bug: 193906385
Test: ran KFENCE test suite with __GFP_DMA on QEMU
Link: https://lore.kernel.org/linux-mm/20210714092222.1890268-1-glider@google.com/


Change-Id: Ie69c6134fc1c3e68238457b146e26355c17f5295
Signed-off-by: Alexander Potapenko <glider@google.com>