arm64: make use of is_compat_task instead of hardcoding this test

Each architecture has its own way to determine if a task is a compat task,
by using is_compat_task in arch_mmap_rnd, it allows more genericity and
then it prepares its moving to mm/.

Link: http://lkml.kernel.org/r/20190730055113.23635-3-alex@ghiti.frSigned-off-by: Alexandre Ghiti <alex@ghiti.fr>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm64/mm/mmap.c

@@ -43,7 +43,7 @@ unsigned long arch_mmap_rnd(void)
 	unsigned long rnd;
 
 #ifdef CONFIG_COMPAT
-	if (test_thread_flag(TIF_32BIT))
+	if (is_compat_task())
 		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
 	else
 #endif