Fix uninitialized varible in zfs_do_userspace()

Fix uninitialized varible in zfs_do_userspace()

When compiling under Debian Lenny with gcc version 4.3.2
(Debian 4.3.2-1.1) the following warning occurs.  To quiet
the warning initialize 'error' to zero.  Newer versions of
gcc correctly determine that this uninitialized varible is
impossible because ZFS_NUM_USERQUOTA_PROPS is known to be
greater than zero.

  cmd/zfs/zfs_main.c:2377: warning: "error" may be
  used uninitialized in this function

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