Enable .zfs/snapshot for 32-bit systems

Originally the .zfs/snapshot directory was disabled for 32-bit systems
because 64-bit inode numbers were not supported. This is no longer
the case and this functionality can be enabled by default.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5347
Closes #2002