btrfsprogs: upgrade to current git master
linux-3.10: backport a fix for "btrfs send"

It has been submitted for inclusion in mainline, so it will probably
make it into 3.11 (or 3.12 as 3.11 is fairly close to release).

It is very local, only affecting people who use the "send" feature.
Without it, send is unstable/unsafe to use incrementally.

It can probably be applied to 3.9 and 3.8 as well, but as I only
tested it against 3.10, so I didn't bother.