namespace: convert fsmount() to FD_PREPARE()

namespace: convert fsmount() to FD_PREPARE()

Christian Brauner <brauner@kernel.org> says:

A variant of the fix sent in [1] was squashed into this commit.

Link: https://lore.kernel.org/20251128035149.392402-1-kartikey406@gmail.com
Reported-by: Deepanshu Kartikey <kartikey406@gmail.com>
Reported-by: syzbot+94048264da5715c251f9@syzkaller.appspotmail.com
Tested-by: syzbot+94048264da5715c251f9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=94048264da5715c251f9
Link: https://patch.msgid.link/20251123-work-fd-prepare-v4-7-b6efa1706cfd@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>