lockdep/mm: Fix might_fault() lockdep check of current->mm->mmap_lock

Turns out that this commit, about 10 years ago:

  9ec2353 ("sched/preempt, mm/fault: Trigger might_sleep() in might_fault() with disabled pagefaults")

... accidentally (and unnessecarily) put the lockdep part of
__might_fault() under CONFIG_DEBUG_ATOMIC_SLEEP=y.

This is potentially notable because large distributions such as
Ubuntu are running with !CONFIG_DEBUG_ATOMIC_SLEEP.

Restore the debug check.

[ mingo: Update changelog. ]

Fixes: 9ec2353 ("sched/preempt, mm/fault: Trigger might_sleep() in might_fault() with disabled pagefaults")
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Peter Zijlstra

mm/memory.c

@@ -6835,10 +6835,8 @@ void __might_fault(const char *file, int line)
 	if (pagefault_disabled())
 		return;
 	__might_sleep(file, line);
-#if defined(CONFIG_DEBUG_ATOMIC_SLEEP)
 	if (current->mm)
 		might_lock_read(&current->mm->mmap_lock);
-#endif
 }
 EXPORT_SYMBOL(__might_fault);
 #endif