btrfs: Simplify update of space_info in __reserve_metadata_bytes()

We don't need an if-else-if chain where we can use a simple OR since
both conditions are performing the same action. The short-circuit for OR
will ensure that if the first condition is true, can_overcommit() is not
called.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>