Set block num to original *count in a case
of retrying allocation in case num < *count

Link: https://lore.kernel.org/r/20191020232326.84881-1-cgxu519@mykernel.netSigned-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>