btrfs: enable shutdown ioctl for non-experimental builds

btrfs: enable shutdown ioctl for non-experimental builds

Although commit 304076527c38 ("btrfs: move shutdown and remove_bdev
callbacks out of experimental features") tries to move both shutdown and
remove_bdev out of experimental features, that commit has only addressed
the super block operation callback, the ioctl one is left untouched.

Fix that missing aspect by also moving shutdown ioctl out of
experimental features.

Since we're here, also add unknown flag detection to reject any
unsupported shutdown flags.

Fixes: 304076527c38 ("btrfs: move shutdown and remove_bdev callbacks out of experimental features")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>