cxgb3: Avoid potential string truncation in desc

cxgb3: Avoid potential string truncation in desc

Builds with W=1 were warning about potential string truncations:

drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c: In function 'cxgb_up':
drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:394:38: warning: '%d' directive output may be truncated writing between 1 and 11 bytes into a region of size between 5 and 20 [-Wformat-truncation=]
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                      ^~
In function 'name_msix_vecs',
    inlined from 'cxgb_up' at drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:1264:3: drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:394:34: note: directive argument in the range [-2147483641, 509]
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                  ^~~~~~~
drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:393:25: note: 'snprintf' output between 3 and 28 bytes into a destination of size 21
  393 |                         snprintf(adap->msix_info[msi_idx].desc, n,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Avoid open-coded %NUL-termination (this code was assuming snprintf
wasn't %NUL terminating when it does -- likely thinking of strncpy),
and grow the size of the string to handle a maximal value.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312100937.ZPZCARhB-lkp@intel.com/
Cc: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231212220954.work.219-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>