nilfs2: fix use-after-free bug of nilfs_root in nilfs_evict_inode()

In the Linux kernel, the following vulnerability has been resolved: nilfs2: fix use-after-free bug of nilfs_root in nilfs_evict_inode() During unmount process of nilfs2, nothing holds nilfs_root structure after nilfs2 detaches its writer in nilfs_detach_log_writer(). However, since nilfs_evict_inode() uses nilfs_root for some cleanup operations, it may cause use-after-free read if inodes are left in "garbage_list" and released by nilfs_dispose_list() at the end of nilfs_detach_log_writer(). Fix this issue by modifying nilfs_evict_inode() to only clear inode without additional metadata changes that use nilfs_root if the file system is degraded to read-only or the writer is detached.