[PATCH V2] Add support for BTRFS raid5/6 to GRUB

[Goffredo Baroncelli]

Hi All,

the aim of this patches set is to provide support for a BTRFS raid5/6
filesystem in GRUB.

The first patch, implements the basic support for raid5/6. I.e this works when
all the disks are present.

The next 4 patches, are preparatory ones.

The 6th patch implements the raid5 recovery for btrfs (i.e. handling the
disappearing of 1 disk).
The 7th patch makes the code for handling the raid6 recovery more generic.
The last one implements the raid6 recovery for btrfs (i.e. handling the
disappearing up to two disks).

I tested the code in grub-emu, and it works (for me) both with all the disks,
and with some disks missing. I checked the crc32 calculated from grub and
from linux and these matched. Finally I checked if the support for md raid6 
still works properly, and it does (with all the drives and with up to 2 drives
missing)

Comments are welcome.

Changelog
v1: initial support for btrfs raid5/6. No recovery allowed
v2: full support for btrfs raid5/6. Recovery allowed
v3: some minor cleanup suggested by Daniel Kiper; reusing the
    original raid6 recovery code of grub


BR
G.Baroncelli