Get rid of mnt_mountpoint abuses in ext4

path to mnt/mnt->mnt_root is no worse than that to
mnt->mnt_parent/mnt->mnt_mountpoint *and* needs no
pinning the sucker down (mnt is not going away and
mnt->mnt_root won't change)

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