ext4: use sbi instead of EXT4_SB(sb) in ext4_update_super()

No behavioral change.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20201216101844.22917-6-jack@suse.cz


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