Fix coverity defects: CID 147654, 147690

coverity scan CID:147654,type: Copy into fixed size buffer

• string operation may write past the end of the fixed-size destination buffer

coverity scan CID:147690,type: Uninitialized scalar variable

• call zfs_prop_get first in case we use sourcetype and share_sourcetype without initialization

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Closes #5253