Btrfs: remove BUG() in btrfs_extent_inline_ref_size

Now that btrfs_get_extent_inline_ref_type() can report if type is a
valid one and all callers can gracefully deal with that, we don't need
to crash here.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>