dma-debug: allow poisoning nonzero allocations

dma-debug: allow poisoning nonzero allocations

Since some dma_alloc_coherent implementations return a zeroed buffer
regardless of whether __GFP_ZERO is passed, there exist drivers which are
implicitly dependent on this and pass otherwise uninitialised buffers to
hardware.  This can lead to subtle and awkward-to-debug issues using those
drivers on different platforms, where nonzero uninitialised junk may for
instance occasionally look like a valid command which causes the hardware
to start misbehaving.  To help with debugging such issues, add the option
to make uninitialised buffers much more obvious.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>