Define MIN_CHUNK_SIZE to replace the hard-coding number.
We need 4k for metadata, so MIN_CHUNK_SIZE should be at least 8k.

Link: https://lore.kernel.org/r/20210528113018.52290-7-jinpu.wang@ionos.comSigned-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>