This function tries locate the right alloc group, where a given physical block
resides, it returns the caller a buffer_head of victim group descriptor, and also
the offset of block in this group, by passing the block number.
Signed-off-by: NTristan Ye <tristan.ye@oracle.com>

It's a relatively complete function to accomplish defragmentation for entire
or partial extent, one journal handle was kept during the operation, it was
logically doing one more thing than ocfs2_move_extent() acutally, yes, it's
claiming the new clusters itself;-)
Signed-off-by: NTristan Ye <tristan.ye@oracle.com>

The moving range of __ocfs2_move_extent() was within one extent always, it
consists following parts:

1. Duplicates the clusters in pages to new_blkoffset, where extent to be moved.

2. Split the original extent with new extent, coalecse the nearby extents if possible.

3. Append old clusters to truncate log, or decrease_refcount if the extent was refcounted.
Signed-off-by: NTristan Ye <tristan.ye@oracle.com>

Ocfs2/move_extents: lock allocators and reserve metadata blocks and data clusters for extents moving.

ocfs2_lock_allocators_move_extents() was like the common ocfs2_lock_allocators(),
to lock metadata and data alloctors during extents moving, reserve appropriate
metadata blocks and data clusters, also performa a best- effort to calculate the
credits for journal transaction in one run of movement.
Signed-off-by: NTristan Ye <tristan.ye@oracle.com>

Adding new files move_extents.[c|h] and fill it with nothing but
only a context structure.
Signed-off-by: NTristan Ye <tristan.ye@oracle.com>