btrfs: use verbose assert at peek_discard_list()

We now have a verbose variant of ASSERT() so that we can print the value
of the block group's discard_index. So use it for better problem analysis
in case the assertion is triggered.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>