The prior patch added support for passing gfp flags through to the
underlying allocators. This patch allows users to pass along gfp flags
(currently only __GFP_NORETRY and __GFP_NOWARN) to the underlying
allocators. This should allow users to decide if they are ok with
failing allocations recovering in a more graceful way.

Additionally, gfp passing was done as additional flags in the previous
patch. Instead, change this to caller passed semantics. GFP_KERNEL is
also removed as the default flag. It continues to be used for internally
caused underlying percpu allocations.

V2:
Removed gfp_percpu_mask in favor of doing it inline.
Removed GFP_KERNEL as a default flag for __alloc_percpu_gfp.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>