debugfs: add the ability to manipulate the extent tree directly

This commit adds the functionality which had previously only been in
the tst_extents command to debugfs.  The debugfs command extent_open
will open extent tree of a particular inode, and enables a series of
commands which will allow the user to interact with the extent tree
directly.  Once the extent tree is closed via extent_open(), these
additional commands will be disabled again.

This commit exports two new functions from lib/ext2fs/extent.c which
had previously been statically defined: ext2fs_extent_node_split() and
ext2fs_extent_goto2().

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/debugfs/ro_debug_cmds.ct b/debugfs/ro_debug_cmds.ct
index 4feb621..8226d1a 100644
--- a/debugfs/ro_debug_cmds.ct
+++ b/debugfs/ro_debug_cmds.ct
@@ -87,5 +87,8 @@
 request do_dump_mmp, "Dump MMP information",
 	dump_mmp;
 
+request do_extent_open, "Open inode for extent manipulation",
+	extent_open, eo;
+
 end;