powerpc/mem: Optimise flush_dcache_icache_hugepage()

flush_dcache_icache_hugepage() is a static function, with
only one caller. That caller calls it when PageCompound() is true,
so bugging on !PageCompound() is useless if we can trust the
compiler a little. Remove the BUG_ON(!PageCompound()).

The number of elements of a page won't change over time, but
GCC doesn't know about it, so it gets the value at every iteration.

To avoid that, call compound_nr() outside the loop and save it in
a local variable.

Whether the page is a HIGHMEM page or not doesn't change over time.

But GCC doesn't know it so it does the test on every iteration.

Do the test outside the loop.

When the page is not a HIGHMEM page, page_address() will fallback on
lowmem_page_address(), so call lowmem_page_address() directly and
don't suffer the call to page_address() on every iteration.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ab03712b70105fccfceef095aa03007de9295a40.1617895813.git.christophe.leroy@csgroup.eu