arm64/mm: mark private VM_FAULT_X defines as vm_fault_t

This patch fixes several sparse warnings for fault.c:

arch/arm64/mm/fault.c:493:24: sparse: warning: incorrect type in return expression (different base types)
arch/arm64/mm/fault.c:493:24: sparse:    expected restricted vm_fault_t
arch/arm64/mm/fault.c:493:24: sparse:    got int
arch/arm64/mm/fault.c:501:32: sparse: warning: incorrect type in return expression (different base types)
arch/arm64/mm/fault.c:501:32: sparse:    expected restricted vm_fault_t
arch/arm64/mm/fault.c:501:32: sparse:    got int
arch/arm64/mm/fault.c:503:32: sparse: warning: incorrect type in return expression (different base types)
arch/arm64/mm/fault.c:503:32: sparse:    expected restricted vm_fault_t
arch/arm64/mm/fault.c:503:32: sparse:    got int
arch/arm64/mm/fault.c:511:24: sparse: warning: incorrect type in return expression (different base types)
arch/arm64/mm/fault.c:511:24: sparse:    expected restricted vm_fault_t
arch/arm64/mm/fault.c:511:24: sparse:    got int
arch/arm64/mm/fault.c:670:13: sparse: warning: restricted vm_fault_t degrades to integer
arch/arm64/mm/fault.c:670:13: sparse: warning: restricted vm_fault_t degrades to integer
arch/arm64/mm/fault.c:713:39: sparse: warning: restricted vm_fault_t degrades to integer

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Min-Hua Chen <minhuadotchen@gmail.com>
Link: https://lore.kernel.org/r/20230502151909.128810-1-minhuadotchen@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>