ANDROID: ext4: don't put symlink in pagecache into highmem

This is a partial backport of 21fc61c73c3903c4c312d0802da01ec2b323d174.

ext4_encrypted_follow_link uses kmap() for cpage
  caddr = kmap(cpage);

_ext4_fname_disk_to_usr calls virt_to_page on the kmapped address.
  _ext4_fname_disk_to_usr()
    ext4_fname_decrypt()
      sg_init_one()
        sg_init_one(&src_sg, iname->name, iname->len);
          sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));

Bug: 71602077
Change-Id: If86e58dd6126dbe5dd6a234d7ffe71bb638a07cd
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jin Qian <jinqian@google.com>