btrfs: force chunk allocation if our global rsv is larger than metadata

btrfs: force chunk allocation if our global rsv is larger than metadata

Nikolay noticed a bunch of test failures with my global rsv steal
patches.  At first he thought they were introduced by them, but they've
been failing for a while with 64k nodes.

The problem is with 64k nodes we have a global reserve that calculates
out to 13MiB on a freshly made file system, which only has 8MiB of
metadata space.  Because of changes I previously made we no longer
account for the global reserve in the overcommit logic, which means we
correctly allow overcommit to happen even though we are already
overcommitted.

However in some corner cases, for example btrfs/170, we will allocate
the entire file system up with data chunks before we have enough space
pressure to allocate a metadata chunk.  Then once the fs is full we
ENOSPC out because we cannot overcommit and the global reserve is taking
up all of the available space.

The most ideal way to deal with this is to change our space reservation
stuff to take into account the height of the tree's that we're
modifying, so that our global reserve calculation does not end up so
obscenely large.

However that is a huge undertaking.  Instead fix this by forcing a chunk
allocation if the global reserve is larger than the total metadata
space.  This gives us essentially the same behavior that happened
before, we get a chunk allocated and these tests can pass.

This is meant to be a stop-gap measure until we can tackle the "tree
height only" project.

Fixes: 0096420adb03 ("btrfs: do not account global reserve in can_overcommit")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>