xfs_mdrestore: Don't rewind source file stream

xfs_mdrestore: Don't rewind source file stream

Today, xfs_mdrestore from stdin will fail if the -i flag is
specified, because it attempts to rewind the stream after
the initial read of the metablock.  This fails, and
results in an abort with "specified file is not a metadata
dump."

Read the metablock exactly once in main(), validate the magic,
print informational flags if requested, and then pass it to
perform_restore() which will then continue the restore process.

Reported-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Marco A Benatto <marco.antonio.780@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>