fs: f2fs: Remove unnecessary checks of SM_I(sbi) in update_general_status()

In fill_super() and put_super(), f2fs_destroy_stats() is called
in prior to f2fs_destroy_segment_manager(), so if current
sbi can still be visited in global stat list, SM_I(sbi) should be
released yet.
For this reason, SM_I(sbi) does not need to be checked in
update_general_status().
Thank Chao Yu for advice.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>