Read spl_hostid module parameter before gethostid()

If spl_hostid is set via module parameter, it's likely different from
gethostid(). Therefore, the userspace tool should read it first before
falling back to gethostid().

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3034