sparc64: refactor code in init_64.c

The sparc64 allmodconfig build broke due to enabling of the
branch_tracer that does some very clever things with
all if conditions. This caused my gcc 3.4.5 to be so confused that
it emitted two warnings:

arch/sparc/mm/init_64.c: In function `update_mmu_cache':
arch/sparc/mm/init_64.c:271: warning: 'pg_flags' might be used uninitialized in this function
arch/sparc/mm/init_64.c:272: warning: 'page' might be used uninitialized in this function

And with -Werror this broke the build.

Refactor code so it:
1) becomes more readable
2) no longer emit a warning with the branch_tracer enabled

The refactoring uses a small helper function (flush_dcache()).

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>