mdrestore: Detect metadump v1 magic before reading the header

mdrestore: Detect metadump v1 magic before reading the header

In order to support both v1 and v2 versions of metadump, mdrestore will have
to detect the format in which the metadump file has been stored on the disk
and then read the ondisk structures accordingly. In a step in that direction,
this commit splits the work of reading the metadump header from disk into two
parts,
1. Read the first 4 bytes containing the metadump magic code.
2. Read the remaining part of the header.

A future commit will take appropriate action based on the value of the magic
code.

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>