preparations to taking MNT_WRITE_HOLD out of ->mnt_flags

We have an unpleasant wart in accessibility rules for struct mount.  There
are per-superblock lists of mounts, used by sb_prepare_remount_readonly()
to check if any of those is currently claimed for write access and to
block further attempts to get write access on those until we are done.

As soon as it is attached to a filesystem, mount becomes reachable
via that list.  Only sb_prepare_remount_readonly() traverses it and
it only accesses a few members of struct mount.  Unfortunately,
->mnt_flags is one of those and it is modified - MNT_WRITE_HOLD set
and then cleared.  It is done under mount_lock, so from the locking
rules POV everything's fine.

However, it has easily overlooked implications - once mount has been
attached to a filesystem, it has to be treated as globally visible.
In particular, initializing ->mnt_flags *must* be done either prior
to that point or under mount_lock.  All other members are still
private at that point.

Life gets simpler if we move that bit (and that's *all* that can get
touched by access via this list) out of ->mnt_flags.  It's not even
hard to do - currently the list is implemented as list_head one,
anchored in super_block->s_mounts and linked via mount->mnt_instance.

As the first step, switch it to hlist-like open-coded structure -
address of the first mount in the set is stored in ->s_mounts
and ->mnt_instance replaced with ->mnt_next_for_sb and ->mnt_pprev_for_sb -
the former either NULL or pointing to the next mount in set, the
latter - address of either ->s_mounts or ->mnt_next_for_sb in the
previous element of the set.

In the next commit we'll steal the LSB of ->mnt_pprev_for_sb as
replacement for MNT_WRITE_HOLD.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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