arm64: mm: Allocate ASIDs in pairs

In preparation for separate kernel/user ASIDs, allocate them in pairs
for each mm_struct. The bottom bit distinguishes the two: if it is set,
then the ASID will map only userspace.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/mmu.h

@@ -17,6 +17,7 @@
 #define __ASM_MMU_H
 
 #define MMCF_AARCH32	0x1	/* mm context flag for AArch32 executables */
+#define USER_ASID_FLAG	(UL(1) << 48)
 
 typedef struct {
 	atomic64_t	id;