mount: separate the flags accessed only under namespace_sem

mount: separate the flags accessed only under namespace_sem

Several flags are updated and checked only under namespace_sem; we are
already making use of that when we are checking them without mount_lock,
but we have to hold mount_lock for all updates, which makes things
clumsier than they have to be.

Take MNT_SHARED, MNT_UNBINDABLE, MNT_MARKED and MNT_UMOUNT_CANDIDATE
into a separate field (->mnt_t_flags), renaming them to T_SHARED,
etc. to avoid confusion.  All accesses must be under namespace_sem.

That changes locking requirements for mnt_change_propagation() and
set_mnt_shared() - only namespace_sem is needed now.  The same goes
for SET_MNT_MARKED et.al.

There might be more flags moved from ->mnt_flags to that field;
this is just the initial set.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>