shmem: use ramfs_kill_sb() for kill_sb method of ramfs-based tmpfs

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

shmem: use ramfs_kill_sb() for kill_sb method of ramfs-based tmpfs

As the ramfs-based tmpfs uses ramfs_init_fs_context() for the

init_fs_context method, which allocates fc->s_fs_info, use ramfs_kill_sb()

to free it and avoid a memory leak.