btrfs: factor btrfs_read_roots() out of open_ctree()

Also, remove the two local variables create_uuid_tree
and check_uuid_tree; we can use the existence of
the uuid root and/or the RESCAN_UUID_TREE flag to
determine what action to take.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>