btrfs: make flush_space take a enum btrfs_flush_state instead of int

I got a automated message from somebody who runs clang against our
kernels and it's because I used the wrong enum type for what I passed
into flush_space, caught by -Wenum-conversion.  Change the argument to
be explicitly the enum we're expecting to make everything consistent.
Maybe eventually gcc will catch errors like this.

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