Btrfs: make BUG_ON() in add_pinned_bytes() an ASSERT()

The value of flags is one of DATA/METADATA/SYSTEM, they must exist at
when add_pinned_bytes is called.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ added changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>