Add IP_RECVFRAGSIZE from Linux 4.10

[Florian Weimer]

* Joseph Myers:

> Linux 4.10 adds a new IP_RECVFRAGSIZE macro to
> include/uapi/linux/in.h.  This patch adds it to glibc's
> sysdeps/unix/sysv/linux/bits/in.h.
>
> Tested for x86_64.
>
> 2017-02-21  Joseph Myers  <joseph@codesourcery.com>
>
> 	* sysdeps/unix/sysv/linux/bits/in.h (IP_RECVFRAGSIZE): New macro.

Looks good to me.  The value is just an int, so no new data type is
needed.