struct mount: relocate MNT_WRITE_HOLD bit

struct mount: relocate MNT_WRITE_HOLD bit

... from ->mnt_flags to LSB of ->mnt_pprev_for_sb.

This is safe - we always set and clear it within the same mount_lock
scope, so we won't interfere with list operations - traversals are
always forward, so they don't even look at ->mnt_prev_for_sb and
both insertions and removals are in mount_lock scopes of their own,
so that bit will be clear in *all* mount instances during those.

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