ext4: refactor ext4_move_extents code base

ext4_move_extents is too complex for review. It has duplicate almost
each function available in the rest of other codebase. It has useless
artificial restriction orig_offset == donor_offset. But in fact logic
of ext4_move_extents is very simple:

Iterate extents one by one (similar to ext4_fill_fiemap_extents)
   ->Iterate each page covered extent (similar to generic_perform_write)
     ->swap extents for covered by page (can be shared with IOC_MOVE_DATA)

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 420c9be..cf3ad75 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2740,10 +2740,15 @@
 				    ext4_lblk_t lblk_start,
 				    ext4_lblk_t lblk_end);
 extern int ext4_find_delalloc_cluster(struct inode *inode, ext4_lblk_t lblk);
+extern ext4_lblk_t ext4_ext_next_allocated_block(struct ext4_ext_path *path);
 extern int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 			__u64 start, __u64 len);
 extern int ext4_ext_precache(struct inode *inode);
 extern int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len);
+extern int ext4_swap_extents(handle_t *handle, struct inode *inode1,
+				struct inode *inode2, ext4_lblk_t lblk1,
+			     ext4_lblk_t lblk2,  ext4_lblk_t count,
+			     int mark_unwritten,int *err);
 
 /* move_extent.c */
 extern void ext4_double_down_write_data_sem(struct inode *first,