debugfs: add the ability to manipulate the extent tree directly

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>