xen/unpopulated-alloc: fix error return code in fill_list()

xen/unpopulated-alloc: fix error return code in fill_list()

Fix to return a negative error code from the error handling case instead
of 0, as done elsewhere in this function.

Fixes: fc1eef363427 ("mm/memremap_pages: convert to 'struct range'")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210508021913.1727-1-thunder.leizhen@huawei.com
Signed-off-by: Juergen Gross <jgross@suse.com>