relay: use kasprintf() instead of fixed buffer formatting

Improve readability and maintainability by replacing a hard coded string
allocation and formatting by using the kasprintf() helper.

It also eliminates the GCC compiler warning (with CONFIG_WERROR=y, which
is default, it becomes an error:

kernel/relay.c:357:42: error: `snprintf' output may be truncated before the last format character [-Werror=format-truncation=]

Link: https://lkml.kernel.org/r/20250317212948.1811176-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>