arm64: mm: EXPORT vabits_user to modules

TASK_SIZE is defined using the vabits_user variable for 64-bit tasks,
so ensure that this variable is exported to modules to avoid the
following build breakage with allmodconfig:

 | ERROR: "vabits_user" [lib/test_user_copy.ko] undefined!
 | ERROR: "vabits_user" [drivers/misc/lkdtm/lkdtm.ko] undefined!
 | ERROR: "vabits_user" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/mmu.c

@@ -53,6 +53,7 @@
 u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
 u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
 u64 vabits_user __ro_after_init;
+EXPORT_SYMBOL(vabits_user);
 
 u64 kimage_voffset __ro_after_init;
 EXPORT_SYMBOL(kimage_voffset);