libext2fs: remove pointless indirection in rbtree bitmaps

libext2fs: remove pointless indirection in rbtree bitmaps

The code was previously allocating a single 4 or 8 byte pointer for
the rcursor and wcursor fields in the ext2fs_rb_private structure;
this added two extra memory allocations (which could fail), and extra
indirections, for no good reason.  Removing the extra indirection also
makes the code more readable, so it's all upside and no downside.

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