fuse: don't keep dead fuse_conn at fuse_fill_super().

syzbot is reporting use-after-free at fuse_kill_sb_blk() [1].
Since sb->s_fs_info field is not cleared after fc was released by
fuse_conn_put() when initialization failed, fuse_kill_sb_blk() finds
already released fc and tries to hold the lock. Fix this by clearing
sb->s_fs_info field after calling fuse_conn_put().

[1] https://syzkaller.appspot.com/bug?id=a07a680ed0a9290585ca424546860464dd9658dbSigned-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+ec3986119086fe4eec97@syzkaller.appspotmail.com>
Fixes: 3b463ae0 ("fuse: invalidation reverse calls")
Cc: John Muir <john@jmuir.com>
Cc: Csaba Henk <csaba@gluster.com>
Cc: Anand Avati <avati@redhat.com>
Cc: <stable@vger.kernel.org> # v2.6.31
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/inode.c

@@ -1193,6 +1193,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 	fuse_dev_free(fud);
  err_put_conn:
 	fuse_conn_put(fc);
+	sb->s_fs_info = NULL;
  err_fput:
 	fput(file);
  err: