net/dns_resolver: use kasprintf + kmemdup_nul to simplify dns_query

Use kasprintf() for descriptions with a query type and kmemdup_nul()
otherwise to simplify dns_query().

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260602071343.962830-2-thorsten.blum@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>