ext4: Remove journal_checksum mount option and enable it by default

There's no real cost for the journal checksum feature, and we should
make sure it is enabled all the time.

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

diff --cc Documentation/filesystems/ext4.txt
Simple merge

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/super.c
Simple merge