vt: selection, handle pending signals in paste_selection

vt: selection, handle pending signals in paste_selection

When pasting a selection to a vt, the task is set as INTERRUPTIBLE while
waiting for a tty to unthrottle. But signals are not handled at all.
Normally, this is not a problem as tty_ldisc_receive_buf receives all
the goods and a user has no reason to interrupt the task.

There are two scenarios where this matters:
1) when the tty is throttled and a signal is sent to the process, it
   spins on a CPU until the tty is unthrottled. schedule() does not
   really echedule, but returns immediately, of course.
2) when the sel_buffer becomes invalid, KASAN prevents any reads from it
   and the loop simply does not proceed and spins forever (causing the
   tty to throttle, but the code never sleeps, the same as above). This
   sometimes happens as there is a race in the sel_buffer handling code.

So add signal handling to this ioctl (TIOCL_PASTESEL) and return -EINTR
in case a signal is pending.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200210081131.23572-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>