nilfs2: fix NULL i_assoc_inode dereference in nilfs_mdt_save_to_shadow_map

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix NULL i_assoc_inode dereference in nilfs_mdt_save_to_shadow_map

The DAT inode's btree node cache (i_assoc_inode) is initialized lazily

during btree operations. However, nilfs_mdt_save_to_shadow_map()

assumes i_assoc_inode is already initialized when copying dirty pages

to the shadow map during GC.

If NILFS_IOCTL_CLEAN_SEGMENTS is called immediately after mount before

any btree operation has occurred on the DAT inode, i_assoc_inode is

NULL leading to a general protection fault.

Fix this by calling nilfs_attach_btree_node_cache() on the DAT inode

in nilfs_dat_read() at mount time, ensuring i_assoc_inode is always

initialized before any GC operation can use it.