zonefs: Fix compilation warning

Avoid the compilation warning "Variable 'ret' is reassigned a value
before the old one has been used." in zonefs_create_zgroup() by setting
ret for the error path only if an error happens.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>