nat: Set file offset back to 0 before writing ip_forward

Kernel versions >= 4.5 have changed the default behavior[1] for file
offset handling on crop/says fs to strict write position handling:

  Respect file position when writing sysctl strings. Multiple writes
  will append to the sysctl value buffer. Anything past the max length
  of the sysctl value buffer will be ignored. Writes to numeric sysctl
  entries must always be at file position 0 and the value must be
  fully contained in the buffer sent in the write syscall.

We first read from /proc/sys/net/ipv4/ip_forward before writing to the
file. Without the lseek to 0 first the write is silentenly ignored.

Bug report from Neil MacLeod <neil@nmacleod.com>.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?id=41662f5cc55335807d39404371cfcbb1909304c4