nilfs2: nilfs_iget_for_gc() returns ERR_PTR

nilfs_iget_for_gc() returns an ERR_PTR() on failure and doesn't return
NULL.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>