spl: cmn_err_once() should be usable in brace-less if else statements

Commit 11913870 (#14567) added cmn_err_once() by #define'ing a
compound statement but failed to consider usage in a single
statement brace-less if else.

Fix the problem by using the common "do {} while (0)" construct.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #14629