xfs_db: exit on invalid magic number

xfs_db: exit on invalid magic number

Fail gracefully and with a useful error message when we detect an
invalid superblock magic in xfs_db.  Add a new -F flag to continue
when seeing the invalid magic number - that way xfs_check and
xfs_metadump can be used on a filesystem that has a corrupted primary
superblock.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>