Introduce GFP_NOWAIT, as an alias for GFP_ATOMIC & ~__GFP_HIGH.

This also changes XFS, which is the only in-tree user of this idiom that I
could find.  The XFS piece is compile-tested only.
Signed-off-by: NJeff Dike <jdike@addtoit.com>
Acked-by: NNathan Scott <nathans@sgi.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>