btrfs: Remove extra parentheses from condition in copy_items()

There is no need for the extra pair of parentheses, remove it. This
fixes the following warning when building with clang:

fs/btrfs/tree-log.c:3694:10: warning: equality comparison with extraneous
  parentheses [-Wparentheses-equality]
                if ((i == (nr - 1)))
                     ~~^~~~~~~~~~~

Also remove the unnecessary parentheses around the substraction.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: David Sterba <dsterba@suse.com>