x86/mm: Fix range check in tlbflush debugfs interface

Since the shift count settable there is used for shifting values
of type "unsigned long", its value must not match or exceed
BITS_PER_LONG (otherwise the shift operations are undefined).

Similarly, the value must not be negative (but -1 must be
permitted, as that's the value used to distinguish the case of
the fine grained flushing being disabled).

Signed-off-by: Jan Beulich <[e-mail]>
Cc: Alex Shi <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/mm/tlb.c

···
 	if (kstrtos8(buf, 0, &shift))
 		return -EINVAL;
 
-	if (shift > 64)
+	if (shift < -1 || shift >= BITS_PER_LONG)
 		return -EINVAL;
 
 	tlb_flushall_shift = shift;