f2fs: fix to not account invalid blocks in get_left_section_blocks()

f2fs: fix to not account invalid blocks in get_left_section_blocks()

w/ LFS mode, in get_left_section_blocks(), we should not account the
blocks which were used before and now are invalided, otherwise those
blocks will be counted as freed one in has_curseg_enough_space(), result
in missing to trigger GC in time.

Cc: stable@kernel.org
Fixes: 249ad438e1d9 ("f2fs: add a method for calculating the remaining blocks in the current segment in LFS mode.")
Fixes: bf34c93d2645 ("f2fs: check curseg space before foreground GC")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>