engines:io_uring: enable support for separate metadata buffer

engines:io_uring: enable support for separate metadata buffer

This patch enables support for separate metadata buffer with
io_uring_cmd ioengine. As we are unaware of metadata size during buffer
allocation, we provide an option md_per_io_size. This option must be
used to specify metadata buffer size for single IO, if namespace is
formatted with a separate metadata buffer.

For the sake of consistency this is the same option as used by SPDK's
external ioengine.

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
Link: https://lore.kernel.org/r/20230814145747.114725-4-ankit.kumar@samsung.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>