e2fsck: avoid out-of-bounds write for very deep extent trees

e2fsck: avoid out-of-bounds write for very deep extent trees

The kernel doesn't support extent trees deeper than 5
(EXT4_MAX_EXTENT_DEPTH).  For this reason we only maintain the extent
tree statistics for 5 levels.  Avoid out-of-bounds writes and reads if
the extent tree is deeper than this.

We keep these statistics to determine whether we should rebuild the
extent tree.  If the extent tree is too deep, we don't need the
statistics because we should always rebuild the it.

Reported-by: Nils Bars <nils.bars@rub.de>
Reported-by: Moritz Schlögel <moritz.schloegel@rub.de>
Reported-by: Nico Schiller <nico.schiller@rub.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>