Create the journal in the middle of the filesystem

Create the journal in the middle of the filesystem

This speeds up access to the journal by eliminating worst-case seeks
from one end of the disk to another, which can be quite common in very
fsync-intensive workloads if the file is located near the end of the
disk, and the journal is located the beginning of the disk.

In addition, this can help eliminate journal fragmentation when
flex_bg is enabled, since the first block group has a large amount of
metadata.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>