fs/btrfs: Fix uninitialized variable

As reported by the Coverity static analysis.
The variable zone is not initialized which
may causes a failed assertion.

Addresses-Coverity: ("Uninitialized variables")
Signed-off-by: Khaled ROMDHANI <khaledromdhani216@gmail.com>

fs/btrfs/zoned.c

@@ -143,6 +143,9 @@ static inline u32 sb_zone_number(int shift, int mirror)
 	case 0: zone = 0; break;
 	case 1: zone = 1ULL << (BTRFS_SB_LOG_FIRST_SHIFT - shift); break;
 	case 2: zone = 1ULL << (BTRFS_SB_LOG_SECOND_SHIFT - shift); break;
+	default:
+		ASSERT(zone);
+		break;
 	}
 
 	ASSERT(zone <= U32_MAX);