libext2fs: set interior tree block goal more intelligently

libext2fs: set interior tree block goal more intelligently

When we're splitting an extent node, try to allocate the new interior
tree block just prior to the first extent in the block we're trying to
split.  The previous logic only set a goal block if we had to split
both the current node and its parent, which is somewhat infrequent.
When that would happen, the goal would start at zero, leading to poor
locality.

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