btrfs/061 has been failing consistently for me recently with a
transaction abort.  We run out of space in the system chunk array, which
means we've allocated way too many system chunks than we need.

Chris added this a long time ago for balance as a poor mans restriping.
If you had a single disk and then added another disk and then did a
balance, update_block_group_flags would then figure out which RAID level
you needed.

Fast forward to today and we have restriping behavior, so we can
explicitly tell the fs that we're trying to change the raid level.  This
is accomplished through the normal get_alloc_profile path.

Furthermore this code actually causes btrfs/061 to fail, because we do
things like mkfs -m dup -d single with multiple devices.  This trips
this check

alloc_flags = update_block_group_flags(fs_info, cache->flags);
if (alloc_flags != cache->flags) {
	ret = btrfs_chunk_alloc(trans, alloc_flags, CHUNK_ALLOC_FORCE);

in btrfs_inc_block_group_ro.  Because we're balancing and scrubbing, but
not actually restriping, we keep forcing chunk allocation of RAID1
chunks.  This eventually causes us to run out of system space and the
file system aborts and flips read only.

We don't need this poor mans restriping any more, simply use the normal
get_alloc_profile helper, which will get the correct alloc_flags and
thus make the right decision for chunk allocation.  This keeps us from
allocating a billion system chunks and falling over.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>