Update headers for Linux 3.15.

Update headers for Linux 3.15.

This patch updates glibc headers for changes / new definitions in
Linux 3.15.  In the course of my review I noticed that
IPV6_PMTUDISC_INTERFACE was absent from glibc despite the inclusion of
IP_PMTUDISC_INTERFACE; I added it along with IP_PMTUDISC_OMIT and
IPV6_PMTUDISC_OMIT.  I did not add FALLOC_FL_NO_HIDE_STALE given the
kernel header comment that it is reserved.

Tested x86_64.

	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h [__USE_GNU]
	(FALLOC_FL_COLLAPSE_RANGE): New macro.
	[__USE_GNU] (FALLOC_FL_ZERO_RANGE): Likewise.
	* sysdeps/unix/sysv/linux/bits/in.h (IP_PMTUDISC_OMIT): Likewise.
	(IPV6_PMTUDISC_INTERFACE): Likewise.
	(IPV6_PMTUDISC_OMIT): Likewise.