libext2fs: when appending to a file, don't split an index block in equal halves

libext2fs: when appending to a file, don't split an index block in equal halves

When we're appending an extent to the end of a file and the index
block is full, don't split the index block into two half-full index
blocks because this leaves us with under utilized index blocks, at
least in the fallocate case.  Instead, copy the last extent from the
full block into the new block.  This isn't perfect utilization, but
there's a lot of work involved in teaching extent.c to be able to goto
a nonexistent node in a newly allocated (and empty) extent block.

This patch does not fix the general problem of keeping the extent tree
balanced.

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