proportions: add @gfp to init functions

Percpu allocator now supports allocation mask.  Add @gfp to
[flex_]proportions init functions so that !GFP_KERNEL allocation masks
can be used with them too.

This patch doesn't make any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>