void (*kill_sb) (struct super_block *);
locking rules:
may block
-get_sb yes
mount yes
kill_sb yes
-->get_sb() returns error or 0 with locked superblock attached to the vfsmount
-(exclusive on ->s_umount).
-->mount() returns ERR_PTR or the root dentry.
+->mount() returns ERR_PTR or the root dentry; its superblock should be locked
+on return.
->kill_sb() takes a write-locked superblock, does all shutdown work on it,
unlocks and drops the reference.