mdrestore: Define and use struct mdrestore

mdrestore: Define and use struct mdrestore

This commit collects all state tracking variables in a new "struct mdrestore"
structure. This is done to collect all the global variables in one place
rather than having them spread across the file. A new structure member of type
"struct mdrestore_ops *" will be added by a future commit to support the two
versions of metadump.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>