tty: hvc: Fix data abort due to race in hvc_open

Potentially, hvc_open() can be called in parallel when two tasks calls
open() on /dev/hvcX. In such a scenario, if the hp->ops->notifier_add()
callback in the function fails, where it sets the tty->driver_data to
NULL, the parallel hvc_open() can see this NULL and cause a memory abort.
Hence, serialize hvc_open and check if tty->private_data is NULL before
proceeding ahead.

The issue can be easily reproduced by launching two tasks simultaneously
that does nothing but open() and close() on /dev/hvcX.
For example:
$ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &

Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Link: https://lore.kernel.org/r/20200428032601.22127-1-rananta@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>