ext4: Completely separate options parsing and sb setup

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 ext4_fill_super() and ext4_remount().

In order to achieve this we have to create new ext4_fill_super() and
ext4_remount() functions which will serve its purpose only until we
actually do convert to the new api (as such they are only temporary for
this patch series) and move the option parsing out of the old function
which will now be renamed to __ext4_fill_super() and __ext4_remount().

There is a small complication in the fact that while the mount option
parsing is going to happen before we get to __ext4_fill_super(), the
mount options stored in the super block itself needs to be applied
first, before the user specified mount options.

So with this patch we're going through the following sequence:

- parse user provided options (including sb block)
- initialize sbi and store s_sb_block if provided
- in __ext4_fill_super()
	- read the super block
	- parse and apply options specified in s_mount_opts
	- check and apply user provided options stored in ctx
	- continue with the regular ext4_fill_super operation

It's not exactly the most elegant solution, but if we still want to
support s_mount_opts we have to do it in this order.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-10-lczerner@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>