fuse: reject a duplicate fd= mount option

fuse_opt_fd() stored the fuse device in ctx->fud and bumped its refcount
unconditionally:

        ctx->fud = fuse_dev_grab(file);

If fd= is given twice (two fsconfig FSCONFIG_SET_FD calls), the second
call overwrites ctx->fud and grabs the new device, while the reference
taken on the first device is never released - a permanent refcount leak
that pins the first fuse_dev until reboot.

Reject a second fd= outright.  ctx is zeroed on allocation, so a non-NULL
ctx->fud reliably means the option was already processed.

Fixes: d42eb23b2e ("fuse: don't require /dev/fuse fd to be kept open during mount")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/inode.c

@@ -791,6 +791,9 @@ static int fuse_opt_fd(struct fs_context *fsc, struct file *file)
 {
 	struct fuse_fs_context *ctx = fsc->fs_private;
 
+	if (ctx->fud)
+		return invalfc(fsc, "Multiple fd specified");
+
 	if (file->f_op != &fuse_dev_operations)
 		return invalfc(fsc, "fd is not a fuse device");
 	/*