btrfs: move shutdown and remove_bdev callbacks out of experimental features

btrfs: move shutdown and remove_bdev callbacks out of experimental features

These two new callbacks have been introduced in v6.19, and it has been
two releases in v7.1.

During that time we have not yet exposed bugs related that two features,
thus it's time to expose them for end users.

It's especially important to expose remove_bdev callback to end users.

That new callback makes btrfs automatically shutdown or go degraded
when a device is missing (depending on if the fs can maintain RW), which
is affecting end users.

We want some feedback from early adopters.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>