MM, NUMA : sys_set_mempolicy() doesnt check if mode < 0

A kernel BUG() is triggered by a call to set_mempolicy() with a negative first argument.
This is because the mode is declared as an int, and the validity check doesnt check < 0 values.
Alternatively, mode could be declared as unsigned int or unsigned long.

Thank you
Eric
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Test program for x86_64:
---------------------------------
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <linux/unistd.h>

#define __NR_set_mempolicy      238
#define __sys_set_mempolicy(mode, nmask, maxnode) _syscall3(int, set_mempolicy, int, mode, unsigned long *, nmask, unsigned long, maxnode)
static __sys_set_mempolicy(mode, nmask, maxnode)

unsigned long nodes = 3;

int main()
{
int ret = set_mempolicy(-6, &nodes, 2);
printf("result=%d errno=%d\n", ret, errno);
return 0;
}


Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>