ext4: Set journal pointer to NULL when journal is released

Set sbi->s_journal to NULL after we call journal_destroy(). This
will be later needed because after journal_destroy() is called,
ext4_clear_inode() can still be called for some inodes (e.g. root
inode) and we'll need to detect there that journal doesn't exists
anymore.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>