XArray: Add calls to might_alloc()

Catch bogus GFP flags deterministically, instead of occasionally
when we actually have to allocate memory.

Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

tools/include: Add (mostly) empty include/linux/sched/mm.h

Now required by liblockdep.

Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
[ Added header guard. ]
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: a.p.zijlstra@chello.nl
Cc: ben@decadent.org.uk
Link: http://lkml.kernel.org/r/20170525130005.5947-17-alexander.levin@verizon.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>