[BUG] can not allocate space for caching data

[Miao Xie <miaox@cn.fujitsu.com>]

Hi, Chris

There is something wrong with this patch:

commit 83a50de97fe96aca82389e061862ed760ece2283
Author: Chris Mason <chris.mason@oracle.com>
Date:   Mon Dec 13 15:06:46 2010 -0500

    Btrfs: prevent RAID level downgrades when space is low
    
    The extent allocator has code that allows us to fill
    allocations from any available block group, even if it doesn't
    match the raid level we've requested.
    
    This was put in because adding a new drive to a filesystem
    made with the default mkfs options actually upgrades the metadata from
    single spindle dup to full RAID1.
    
    But, the code also allows us to allocate from a raid0 chunk when we
    really want a raid1 or raid10 chunk.  This can cause big trouble because
    mkfs creates a small (4MB) raid0 chunk for data and metadata which then
    goes unused for raid1/raid10 installs.
    
    The allocator will happily wander in and allocate from that chunk when
    things get tight, which is not correct.
    
    The fix here is to make sure that we provide duplication when the
    caller has asked for it.  It does all the dups to be any raid level,
    which preserves the dup->raid1 upgrade abilities.
    
    Signed-off-by: Chris Mason <chris.mason@oracle.com>

Btrfs has added the space of single chunks and raid0 chunks into the space
information, so when we use btrfs_check_data_free_space() to check if there
is some space for storing file data, this function may return true. So we
write the data into the cache successfully. But, the extent allocator can
not allocate any space to store that cached data, and then the file system
panic.

I think we subtract that space from the space information, or split the space
information into two types, one is used to manage the chunks with duplication,
the other manages the other chunks.