f2fs: remove redundant code for gc condition

Remove the redundant code and use local variant as the
argument directly. Make it more human-readable.

Signed-off-by: duguowei <duguowei@xiaomi.com>
[Jaegeuk Kim: make code neat]
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>