fanotify: convert fanotify_init() to FD_PREPARE()

fanotify: convert fanotify_init() to FD_PREPARE()

Christian Brauner <brauner@kernel.org> says:

The fix sent in [1] was squashed into this commit.

Link: https://lore.kernel.org/20251127201618.2115275-1-kuniyu@google.com
Reported-by: syzbot+321168dfa622eda99689@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/6928b121.a70a0220.d98e3.0110.GAE@google.com
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251123-work-fd-prepare-v4-8-b6efa1706cfd@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>