32-bit compat, hostid_read()

Explicitly cast the sizeof in hostid_read() to prevent the
following compiler warning on 32-bit systems.

module/spl/spl-generic.c:490:10: error: format '%lu' expects
argument of type 'long unsigned int', but argument 4 has type
'unsigned int' [-Werror=format]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>