LU-1402 libcfs: CFS_ALLOC_HIGH is __GFP_HIGHMEM

According to the comment above CFS_ALLOC_HIGH, the flag
allows allocator to return memory in the highmen zone, but
cfs_alloc_flags_to_gfp() incorrectly converts it to
__GFP_HIGH, which enables the use of emergency pools.

__GFP_HIGHMEM should be used instead. Also it makes sense to rename
CFS_ALLOC_HIGH to CFS_ALLOC_HIGHMEM to avoid further confusion.

Signed-off-by: Isaac Huang <Isaac_Huang@xyratex.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I088f36faec1fe22902065d680730731f8deab5d2
Reviewed-on: http://review.whamcloud.com/2732
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Liang Zhen <liang@whamcloud.com>