btrfs: move the snapshot drop related prototypes to extent-tree.h

These belong in extent-tree.h, they were missed because they were not
grouped with the other extent-tree.c prototypes.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>