f2fs: improve definition of statistic macros

With a recent addition of f2fs_lookup_extent_tree(), we get a warning about
the use of empty macros:

fs/f2fs/extent_cache.c: In function 'f2fs_lookup_extent_tree':
fs/f2fs/extent_cache.c:358:32: error: suggest braces around empty body in an 'else' statement [-Werror=empty-body]
   stat_inc_rbtree_node_hit(sbi);

A good way to avoid the warning and make the code more robust is to define
all no-op macros as 'do { } while (0)'.

Fixes: 54c2258c ("f2fs: extract rb-tree operation infrastructure")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reivewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>