btrfs: increase ->free_chunk_space in btrfs_grow_device

btrfs: increase ->free_chunk_space in btrfs_grow_device

My overcommit patch exposed a bug with btrfs/177 [1].  The problem here is
that when we grow the device we're not adding to ->free_chunk_space, so
subsequent allocations can cause ->free_chunk_space to wrap, which
causes problems in can_overcommit because we add this to ->total_bytes,
which causes the counter to wrap and gives us an unexpected ENOSPC.

Fix this by properly updating ->free_chunk_space with the new available
space in btrfs_grow_device.

[1] First version of the fix:
    https://lore.kernel.org/linux-btrfs/b97e47ce0ce1d41d221878de7d6090b90aa7a597.1695065233.git.josef@toxicpanda.com/

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>