mm: fix noisy sparse warning in LIBCFS_ALLOC_PRE()

Running sparse on drivers/staging/lustre results in dozens of warnings:
include/linux/gfp.h:281:41: warning: odd constant _Bool cast (400000
becomes 1)

Use "!!" to explicitly convert to bool and get rid of the warning.
Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -278,7 +278,7 @@ static inline int gfpflags_to_migratetype(const gfp_t gfp_flags)
 
 static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
 {
-	return (bool __force)(gfp_flags & __GFP_DIRECT_RECLAIM);
+	return !!(gfp_flags & __GFP_DIRECT_RECLAIM);
 }
 
 #ifdef CONFIG_HIGHMEM