ext4: use sbi in ext4_orphan_{add|del}()

Use sbi pointer consistently in ext4_orphan_del() instead of opencoding
it sometimes. Also ext4_orphan_add() uses EXT4_SB(sb) often so create
sbi variable for it as well and use it.

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