Fix coverity defects: CID 147639

When array is passed as a parameter it degenerates into a
pointer so the sizeof(path) in is_shorthand_path() and always
get return value of 8, instead of the string length we want.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Closes #5198