fs: fix UAF/GPF bug in nilfs_mdt_destroy

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

fs: fix UAF/GPF bug in nilfs_mdt_destroy

In alloc_inode, inode_init_always() could return -ENOMEM if

security_inode_alloc() fails, which causes inode->i_private

uninitialized. Then nilfs_is_metadata_file_inode() returns

true and nilfs_free_inode() wrongly calls nilfs_mdt_destroy(),

which frees the uninitialized inode->i_private

and leads to crashes(e.g., UAF/GPF).

Fix this by moving security_inode_alloc just prior to

this_cpu_inc(nr_inodes)