libext2fs: fix rbtree backend for extent lengths greater than 2^32

libext2fs: fix rbtree backend for extent lengths greater than 2^32

For a completely full filesystem with more than 2^32 blocks, the
rbtree bitmap backend can assemble an extent of used blocks which is
longer than 2^32.  If it does, it will overflow ->count, and corrupt
the rbtree for the bitmaps.

Discovered by completely filling a 32T filesystem using fallocate, and
then observing debugfs, dumpe2fs, and e2fsck all behaving badly.

(Note that filling with only 31 x 1T files did not show the problem,
because freespace was fragmented enough that there was no sufficiently
long range of used blocks.)

Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>