alpha: Don't export SOCK_NONBLOCK to user space.

Currently we export SOCK_NONBLOCK to user space but that conflicts with
the definition from glibc leading to compilation errors in user programs
(e.g.  see Debian bug #658460).

The generic socket.h restricts the definition of SOCK_NONBLOCK to the
kernel, as does the MIPS specific socket.h, so let's do the same on
Alpha.

Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Acked-by: Matt Turner <mattst88@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>