Fix coverity defects: CID 147643, 152204, 49339

CID 147643: Type: String not null terminated

• make sure that the string is null terminated before strlen and fprintf.

CID 152204: Type: Copy into fixed size buffer

• since strlcpy isn't availabe here, use strncpy and terminate the string manually.

CID 49339: Type: Buffer not null terminated

• since strlcpy isn't availabe here, terminate the string manually before fprintf.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Closes #5283