Linux 5.0 compat: Use totalhigh_pages()

Linux 5.0 compat: Use totalhigh_pages()

Linux kernel commit ca79b0c211af63fa3276f0e3fd7dd9ada2439839
"mm: convert totalram_pages and totalhigh_pages variables to atomic"

replaced `totalhigh_pages` with an inline function `totalhigh_pages()`.
This broke compilation on IA32, etc, as ZoL uses `totalhigh_pages`
on archs with highmem. Confirmed on Fedora 30 (5.0.9-301.fc30.i686).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8677
Closes #8701