debugfs: Add the new command dump_extents and extent the stat command

debugfs: Add the new command dump_extents and extent the stat command

Extend the stat command to display more detailed extent information if
the file uses extent mapping instead of displaying the block map using
the block_iterate funtion.

Add the command dump_extents which displays even more detailed
information about an inode's extent tree.

This commit is an extension of a patch from Curt Wohlgemuth.

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>