btrfs: fix uninitialized variable warning in btrfs_sb_log_location

We only have 3 possible mirrors, and we have ASSERT()'s to make sure
we're not passing in an invalid super mirror into this function, so
technically this value isn't uninitialized.  However
-Wmaybe-uninitialized will complain, so set it to U64_MAX so if we don't
have ASSERT()'s turned on it'll error out later on when it see's the
zone is beyond our maximum zones.
Reviewed-by: NNaohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/zoned.c

@@ -160,7 +160,7 @@ static int sb_write_pointer(struct block_device *bdev, struct blk_zone *zones,
  */
 static inline u32 sb_zone_number(int shift, int mirror)
 {
-	u64 zone;
+	u64 zone = U64_MAX;
 
 	ASSERT(mirror < BTRFS_SUPER_MIRROR_MAX);
 	switch (mirror) {