libext2fs: fix endian handling error; reduce fragmentation some

libext2fs: fix endian handling error; reduce fragmentation some

If we're going to read the "nr - 1" entry in an indirect block for use
as a "goal" input to the block allocator, we need to byteswap the
entry.  While we're at it, if we're allocating blocks for the zeroth
entry in the indirect block, we might as well use the indirect block
as the starting point to try to reduce fragmentation.

(d_fallocate_blkmap will test this...)

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