f2fs: fix to update f2fs_super_block fields under sb_lock

Fields in struct f2fs_super_block should be updated under coverage
of sb_lock, fix to adjust update_sb_metadata() for that rule.

Fixes: 04f0b2eaa3b3 ("f2fs: ioctl for removing a range from F2FS")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>