crimson/os: use compile-time validation

crimson/os: use compile-time validation

libfmt does compile-time format argument validation of the format string
and the argument when the the user-defined literal is used. but the
downside is that the formatter materialize the whole formatted string
into a std::string, before printing them argument into seastar's log buffer
inserter. presumably, the inserter would be more efficient in
comparision to the pre-format approach. so this validation is only
enabled for non NDEBUG build. so it is able to help us to identify
errors like

DEBUG("{} {}", 1, 2, 3)

Signed-off-by: Kefu Chai <kchai@redhat.com>