vlock: ensure locked VT is active when locking all consoles

vlock -a is documented to lock all console sessions and disable VT
switching, telling the user that the entire console display is locked.
This guarantee relies entirely on holding VT_PROCESS mode on the active
VT because vlock refuses switch requests only when its own VT is in the
foreground (change_console() in the kernel consults only fg_console).

If a VT switch occurs before VT_SETMODE takes effect during startup,
or if vlock -a is invoked on a background VT (e.g. "chvt 2; vlock -a" or
via an idle timer), VT_SETMODE succeeds on the background terminal
without protecting anything. Another console remains active with
unrestricted switching, while vlock reports success and falsely claims
the display is locked.

Resolve the current VT number via TIOCGDEV and pull the terminal to
the foreground using VT_ACTIVATE and VT_WAITACTIVE after entering
VT_PROCESS mode. If the VT is already active, the kernel performs no
switch. If activation or waiting fails, restore the previous VT mode and
fail rather than claiming a lock that is not enforced.

Reported-by: Vitezslav Crhonek <vcrhonek@redhat.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>