kdump: use print_mask_arg0 appropriately

Some callers are using print_mask_arg() when they should be using
print_mask_arg0(); the latter should be used when all flags are optional
and there's not a flag to be decoded with a 0-mask.  This turns:

nmount(0x6991e009000,0x8,0<><invalid>0)

into:

nmount(0x6991e009000,0x8,0)

Reviewed by:	jhb
Differential Revision:	https://reviews.freebsd.org/D43007

usr.bin/kdump/kdump.c

@@ -922,7 +922,8 @@ ktrsyscall_freebsd(struct ktr_syscall *ktr, register_t **resip,
 				print_number(ip, narg, c);
 				print_number(ip, narg, c);
 				putchar(',');
-				print_mask_arg(sysdecode_close_range_flags, *ip);
+				print_mask_arg0(sysdecode_close_range_flags,
+				    *ip);
 				ip += 3;
 				narg -= 3;
 				break;
@@ -988,14 +989,14 @@ ktrsyscall_freebsd(struct ktr_syscall *ktr, register_t **resip,
 				print_number(ip, narg, c);
 				print_number(ip, narg, c);
 				putchar(',');
-				print_mask_arg(sysdecode_mount_flags, *ip);
+				print_mask_arg0(sysdecode_mount_flags, *ip);
 				ip++;
 				narg--;
 				break;
 			case SYS_unmount:
 				print_number(ip, narg, c);
 				putchar(',');
-				print_mask_arg(sysdecode_mount_flags, *ip);
+				print_mask_arg0(sysdecode_mount_flags, *ip);
 				ip++;
 				narg--;
 				break;
@@ -1453,7 +1454,7 @@ ktrsyscall_freebsd(struct ktr_syscall *ktr, register_t **resip,
 				print_number(ip, narg, c);
 				print_number(ip, narg, c);
 				putchar(',');
-				print_mask_arg(sysdecode_mount_flags, *ip);
+				print_mask_arg0(sysdecode_mount_flags, *ip);
 				ip++;
 				narg--;
 				break;