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: c0c77d8f ("xsk: add user memory registration support sockopt")
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>