super: massage wait event mechanism

We're currently using two separate helpers wait_born() and wait_dead()
when we can just all do it in a single helper super_load_flags(). We're
also acquiring the lock before we check whether this superblock is even
a viable candidate. If it's already dying we don't even need to bother
with the lock.

Link: https://lore.kernel.org/r/20231127-vfs-super-massage-wait-v1-1-9ab277bfd01a@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>