Given a physical cpos and length, increment the refcount
in the tree. If the extent has not been seen before, a refcount
record is created for it. Refcount records may be merged or
split by this operation.
Signed-off-by: NTao Ma <tao.ma@oracle.com>

Add basic refcount tree root operation.
Signed-off-by: NTao Ma <tao.ma@oracle.com>

Implement locking around struct ocfs2_refcount_tree.  This protects
all read/write operations on refcount trees.  ocfs2_refcount_tree
has its own lock and its own caching_info, protecting buffers among
multiple nodes.

User must call ocfs2_lock_refcount_tree before his operation on
the tree and unlock it after that.

ocfs2_refcount_trees are referenced by the block number of the
refcount tree root block, So we create an rb-tree on the ocfs2_super
to look them up.
Signed-off-by: NTao Ma <tao.ma@oracle.com>

refcount tree should use its own caching info so that when
we downconvert the refcount tree lock, we can drop all the
cached buffer head.
Signed-off-by: NTao Ma <tao.ma@oracle.com>

Signed-off-by: NTao Ma <tao.ma@oracle.com>