[PATCH V6] Add support for BTRFS raid5/6 to GRUB

[Goffredo Baroncelli <kreijack@gmail.com>]

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 5 patches, are preparatory ones.

The 7th patch implements the raid5 recovery for btrfs (i.e. handling the
disappearing of 1 disk).
The 8th 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 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 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
v4: Several spell fix; better description of the RAID layout
    in btrfs, and the variables which describes the stripe
    positioning; split the patch #5 in two (#5 and #6)
v5: Several spell fix; improved code comment in patch #1, small
    clean up in the code
v6: Small cleanup; improved the wording in the RAID6 layout
    description; in the function raid6_recover_read_buffer() avoid
    a unnecessary memcpy in case of invalid data;