mesa: properly report the length of truncated log messages

_mesa_log_msg must provide the length of the string passed into the
KHR_debug api.  When the string formatted by _mesa_gl_vdebugf exceeds
MAX_DEBUG_MESSAGE_LENGTH, the length is incorrectly set to the number
of characters that would have been written if enough space had been
available.

Fixes: 30256805
       ("mesa: Add support for GL_ARB_debug_output with dynamic ID allocation.")

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
(cherry picked from commit eb1a869a)