f2fs: fix to destroy sbi->post_read_wq in error path of f2fs_fill_super()

In error path of f2fs_fill_super(), this patch fixes to call
f2fs_destroy_post_read_wq() once if we fail in f2fs_start_ckpt_thread().

Fixes: 261eeb9c ("f2fs: introduce checkpoint_merge mount option")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>