Change one printf() to log().

As noted in kern/159780, printf() is not very jail-friendly, since it can't be easily monitored by jail management tools. This patch reports an error via log() instead, which, if nobody is watching the log file, still prints to the console.

Approved by: mentor (rwatson)
Submitted by: Eugene Grosbein <eugen@eg.sd.rdtc.ru>
MFC after: 5 days