xsk: fix 64-bit division

i386 builds report:
net/xdp/xdp_umem.o: In function `xdp_umem_reg':
xdp_umem.c:(.text+0x47e): undefined reference to `__udivdi3'

This fix uses div_u64 instead of the GCC built-in.

Fixes: c0c77d8fb787 ("xsk: add user memory registration support sockopt")
Signed-off-by: Björn Töpel <[e-mail]>
Reported-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

net/xdp/xdp_umem.c

···
 	if ((addr + size) < addr)
 		return -EINVAL;
 
-	nframes = size / frame_size;
+	nframes = (unsigned int)div_u64(size, frame_size);
 	if (nframes == 0 || nframes > UINT_MAX)
 		return -EINVAL;