Add loader for internal journal (we need to update mkfs to support this)

For internal journal we need enhance mkfs to use ext2fs lib, that will
allow us to create journal (with proper header).
Implement journal_path option
simple-test: mount fs with journal
Actually load journal by calling jbd2_journal_load()
Handle bdevs for journal_path= mount flag
simple-test: use losetup+journal_path=/dev/loopX for mounting with journal
simple-test: create journal with 4k block size

This must avoid next error:
[    6.694339] Journal device is: unknown-block(7,0)
[    6.695620] JBD2: no valid journal superblock found
Use max available journal size with jbd2_journal_init_dev()

Avoid next error:
JBD2: journal file too short
Add jbd2 {get_write_access,dirty_metadata} into simplefs_write()