ext4: improve code readability in ext4_iget()

Merge the duplicated complex conditions to improve code readability.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>