mm/vmalloc: add a helper to optimize vmalloc allocation gfps

mm/vmalloc: add a helper to optimize vmalloc allocation gfps

vm_area_alloc_pages() attempts to use different gfp flags as a way to
optimize allocations.  This has been done inline which makes things harder
to read.

Add a helper function to make the code more readable.

Link: https://lkml.kernel.org/r/20251121094405.40628-3-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Acked-by: SeongJae Park <sj@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>