fs/ext2: bump journal overhead to 1300 blocks

A quick test with a ~3.5MB ext4 filesystem shows that 1081 blocks isn't
enough:

tune2fs 1.42.7 (21-Jan-2013)

Please run e2fsck on the filesystem.

Creating journal inode:
Journal size too big for filesystem.

So bump it a bit. Overestimating the journal size is probably not really
a big deal for the kind of systems using ext3/4 anyway.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>