f2fs: avoid BG_GC in f2fs_balance_fs

If many threads hit has_not_enough_free_secs() in f2fs_balance_fs() at the same
time, all the threads would do FG_GC or BG_GC.
In this critical path, we totally don't need to do BG_GC at all.
Let's avoid that.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>