btrfs: fix uninitialized variable warnings

There are some warnings on older compilers (gcc 10, 7) or non-x86_64
architectures (aarch64).  As btrfs wants to enable -Wmaybe-uninitialized
by default, fix the warnings even though it's not necessary on recent
compilers (gcc 12+).

../fs/btrfs/volumes.c: In function ‘btrfs_init_new_device’:
../fs/btrfs/volumes.c:2703:3: error: ‘seed_devices’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
 2703 |   btrfs_setup_sprout(fs_info, seed_devices);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

../fs/btrfs/send.c: In function ‘get_cur_inode_state’:
../include/linux/compiler.h:70:32: error: ‘right_gen’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   70 |   (__if_trace.miss_hit[1]++,1) :  \
      |                                ^
../fs/btrfs/send.c:1878:6: note: ‘right_gen’ was declared here
 1878 |  u64 right_gen;
      |      ^~~~~~~~~
Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -1875,7 +1875,7 @@ static int get_cur_inode_state(struct send_ctx *sctx, u64 ino, u64 gen,
 	int left_ret;
 	int right_ret;
 	u64 left_gen;
-	u64 right_gen;
+	u64 right_gen = 0;
 	struct btrfs_inode_info info;
 
 	ret = get_inode_info(sctx->send_root, ino, &info);

fs/btrfs/volumes.c

@@ -2617,7 +2617,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 	struct block_device *bdev;
 	struct super_block *sb = fs_info->sb;
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
-	struct btrfs_fs_devices *seed_devices;
+	struct btrfs_fs_devices *seed_devices = NULL;
 	u64 orig_super_total_bytes;
 	u64 orig_super_num_devices;
 	int ret = 0;