btrfs-progs: Refactor find_next_chunk to get rid of parameter root and objectid

btrfs-progs: Refactor find_next_chunk to get rid of parameter root and objectid

Function find_next_chunk() is used to find next chunk start position,
which should only do search on chunk tree and objectid is set to
BTRFS_FIRST_CHUNK_TREE_OBJECTID.

So refactor the parameter list to get rid of @root, which should be
obtained from fs_info->chunk_root, and @objectid, which is set to
BTRFS_FIRST_CHUNK_TREE_OBJECTID.

Signed-off-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>