Btrfs: handle a bogus chunk tree nicely

If you restore a btrfs-image file system and try to mount that file system we'll
panic.  That's because btrfs-image restores and just makes one big chunk to
envelope the whole disk, since they are really only meant to be messed with by
our btrfs-progs.  So fix up btrfs_rmap_block and the callers of it for mount so
that we no longer panic but instead just return an error and fail to mount.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>