f2fs: do not stop GC when requiring a free section

The f2fs_gc uses a bitmap to indicate pinned sections, but when disabling
chckpoint, we call f2fs_gc() with NULL_SEGNO which selects the same dirty
segment as a victim all the time, resulting in checkpoint=disable failure,
for example. Let's pick another one, if we fail to collect it.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>