super: remove bd_fsfreeze_sb

super: remove bd_fsfreeze_sb

Remove bd_fsfreeze_sb as it's now unused and can be removed. Also move
bd_fsfreeze_count down to not have it weirdly placed in the middle of
the holder fields.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-7-599c19f4faac@kernel.org
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>