abd_os: remove redundant refcount creation for abd_children

Refcount creation for abd_zero_scatter->abd_children is redundant in
abd_alloc_zero_scatter, as it has been done in abd_init_struct.

In addition, abd_children is undefined when ZFS_DEBUG is disabled, the
reference of abd_children in abd_alloc_zero_scatter breaks build of
libzpool when ZFS_DEBUG is disabled.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Ping Huang <huangping@smartx.com>
Closes #13429