mm: correct ALLOC_WMARK_MIN flag check for atomic allocations

ALLOC_WMARK_[MIN|LOW|HIGH] are the indexes into the zones->_watermark
array. Use ALLOC_WMARK_MASK to get a particular water mark alloc flag.
Correct this logic for atomic order-0 allocations watermark check.

Change-Id: I5a74abd579df2ddcca63fdd99a090ca83ccd481c
Fixes: 6b274863 ("mm: ignore boosting for min watermark")
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>