fs: add a method to shut down the file system

Add a new ->shutdown super operation that can be used to tell the file
system to shut down, and call it from newly created holder ops when the
block device under a file system shuts down.

This only covers the main block device for "simple" file systems using
get_tree_bdev / mount_bdev.  File systems their own get_tree method
or opening additional devices will need to set up their own
blk_holder_ops.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Acked-by: Dave Chinner <[email protected]>
Reviewed-by: Dave Chinner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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