int grab_super (struct super_block * s);
Tries to acquire an active reference. grab_super is used when we had just found a superblock in super_blocks or fs_type->fs_supers and want to turn it into a full-blown active reference. grab_super is called with sb_lock held and drops it. Returns 1 in case of success, 0 if we had failed (superblock contents was already dead or dying when grab_super had been called).