vfs: coredumping fix (CVE-2007-6206)

fix: http://bugzilla.kernel.org/show_bug.cgi?id=3043



only allow coredumping to the same uid that the coredumping
task runs under.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>