sparc64: fix build regession

Commit ea1e7ed3 triggers build regression on sparc64.

include/linux/mm.h:1391:2: error: implicit declaration of function 'pgtable_cache_init' [-Werror=implicit-function-declaration]
arch/sparc/include/asm/pgtable_64.h:978:13: error: conflicting types for 'pgtable_cache_init' [-Werror]

It happens due headers include loop:

<linux/mm.h> -> <asm/pgtable.h> -> <asm/pgtable_64.h> ->
	<asm/tlbflush.h> -> <asm/tlbflush_64.h> -> <linux/mm.h>

Let's drop <linux/mm.h> include from asm/tlbflush_64.h.
Build tested with allmodconfig.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>