bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

When compiling the bcachefs-tools, the following compilation warning
is reported:
    libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
    libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      915 |  snapshot->v.tree = cpu_to_le32(tree_id);
    libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
      903 |  u32 tree_id;
       |      ^~~~~~~

This is a false alert, because @tree_id is changed in
bch2_snapshot_tree_create after it returns 0. And if this function
returns other value, @tree_id wouldn't be used. Thus there should
be nothing wrong in logical.

Although the report itself is a false alert, we can still make it more
explicit by setting the initial value of @tree_id to 0 (an invalid
tree ID).

Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>