f2fs: check number of blocks in a current section

In cfd66bb7 ("f2fs: fix deadloop in foreground GC"), we needed to check
the number of blocks in a section instead of the segment.

Fixes: cfd66bb7 ("f2fs: fix deadloop in foreground GC")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>