Btrfs: Avoid using __GFP_HIGHMEM with slab allocator

btrfs_releasepage may call kmem_cache_alloc indirectly,
and provide same GFP flags it gets to kmem_cache_alloc.
So it's possible to use __GFP_HIGHMEM with the slab
allocator.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>