fs: remove dead check

Above we call super_lock_excl() which waits until the superblock is
SB_BORN and since SB_BORN is never unset once set this check can never
fire. Plus, we also hold an active reference at this point already so
this superblock can't even be shutdown.

Link: https://lore.kernel.org/r/20231104-vfs-multi-device-freeze-v2-1-5b5b69626eac@kernel.org


Tested-by: Chandan Babu R <chandanbabu@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>