bnxt: Do not read past the end of test names

Test names were being concatenated based on a offset beyond the end of
the first name, which tripped the buffer overflow detection logic:

 detected buffer overflow in strnlen
 [...]
 Call Trace:
 bnxt_ethtool_init.cold+0x18/0x18

Refactor struct hwrm_selftest_qlist_output to use an actual array,
and adjust the concatenation to use snprintf() rather than a series of
strncat() calls.

Reported-by: Niklas Cassel <Niklas.Cassel@wdc.com>
Link: https://lore.kernel.org/lkml/Y8F%2F1w1AZTvLglFX@x1-carbon/
Tested-by: Niklas Cassel <Niklas.Cassel@wdc.com>
Fixes: eb513658

 ("bnxt_en: Add basic ethtool -t selftest support.")
Cc: Michael Chan <michael.chan@broadcom.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>