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>