libext2fs: add a way to check the theoretical maximum extent tree depth

libext2fs: add a way to check the theoretical maximum extent tree depth

Add an API so that client programs can discover a reasonable maximum
extent tree depth.  This will eventually be used by e2fsck as one of
the criteria to decide if an extent-based file should have its extent
tree rebuilt.

Turn some related magic numbers into constants while we're at it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>