mm/vmalloc: be more explicit about supported gfp flags

mm/vmalloc: be more explicit about supported gfp flags

The core of the vmalloc allocator __vmalloc_area_node doesn't say
anything about gfp mask argument.  Not all gfp flags are supported
though.  Be more explicit about constraints.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Neil Brown <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Uladzislau Rezki <[email protected]>
Cc: Ilya Dryomov <[email protected]>
Cc: Jeff Layton <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>