futex: Move futex exit handling into futex code

The futex exit handling is #ifdeffed into mm_release() which is not pretty
to begin with. But upcoming changes to address futex exit races need to add
more functionality to this exit code.

Split it out into a function, move it into futex code and make the various
futex exit functions static.

Preparatory only and no functional change.

Folded build fix from Borislav.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.049705556@linutronix.de

include/linux/compat.h

@@ -410,8 +410,6 @@ struct compat_kexec_segment;
 struct compat_mq_attr;
 struct compat_msgbuf;
 
-extern void compat_exit_robust_list(struct task_struct *curr);
-
 #define BITS_PER_COMPAT_LONG    (8*sizeof(compat_long_t))
 
 #define BITS_TO_COMPAT_LONGS(bits) DIV_ROUND_UP(bits, BITS_PER_COMPAT_LONG)