nilfs2: use iget5_locked to get inode

This uses iget5_locked instead of iget_locked so that gc cache can
look up inodes with an inode number and an optional checkpoint number.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

diff --cc fs/nilfs2/inode.c
Simple merge

diff --cc fs/nilfs2/super.c
Simple merge