f2fs: reduce searching region of segmap when free section

In __set_test_and_free we will check whether all segment are free in one section
When free one segment, in order to set section to free status.
But the searching region of segmap is from start segno to last segno of f2fs,
it's not necessary. So let's just only check all segment bitmap of target
section.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>