f2fs: separate the options parsing and options checking

The new mount api separates option parsing and super block setup
into two distinct steps and so we need to separate the options
parsing out of the parse_options().

In order to achieve this, here we handle the mount options with
three steps:
  - Firstly, we move sb/sbi out of handle_mount_opt.
    As the former patch introduced f2fs_fs_context, so we record
    the changed mount options in this context. In handle_mount_opt,
    sb/sbi is null, so we should move all relative code out of
    handle_mount_opt (thus, some check case which use sb/sbi should
    move out).
  - Secondly, we introduce the some check helpers to keep the option
    consistent.
    During filling superblock period, sb/sbi are ready. So we check
    the f2fs_fs_context which holds the mount options base on sb/sbi.
  - Thirdly, we apply the new mount options to sb/sbi.
    After checking the f2fs_fs_context, all changed on mount options
    are valid. So we can apply them to sb/sbi directly.

After do these, option parsing and super block setting have been
decoupled. Also it should have retained the original execution
flow.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
[sandeen: forward port, minor fixes and updates]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[hongbo: minor fixes]
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>