kdb: Fix a potential buffer overflow in kdb_local()

kdb: Fix a potential buffer overflow in kdb_local()

When appending "[defcmd]" to 'kdb_prompt_str', the size of the string
already in the buffer should be taken into account.

An option could be to switch from strncat() to strlcat() which does the
correct test to avoid such an overflow.

However, this actually looks as dead code, because 'defcmd_in_progress'
can't be true here.
See a more detailed explanation at [1].

[1]: https://lore.kernel.org/all/CAD=FV=WSh7wKN7Yp-3wWiDgX4E3isQ8uh0LCzTmd1v9Cg9j+nQ@mail.gmail.com/

Fixes: 5d5314d6795f ("kdb: core for kgdb back end (1 of 2)")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Douglas Anderson <dianders@chromium.org>