fcntl: make F_GETOWN(EX) return 0 on dead owner task

Currently there is no way to differentiate the file with alive owner
from the file with dead owner but pid of the owner reused. That's why
CRIU can't actually know if it needs to restore file owner or not,
because if it restores owner but actual owner was dead, this can
introduce unexpected signals to the "false"-owner (which reused the
pid).

Let's change the api, so that F_GETOWN(EX) returns 0 in case actual
owner is dead already. This comports with the POSIX spec, which
states that a PID of 0 indicates that no signal will be sent.

Cc: Jeff Layton <jlayton@kernel.org>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>