btrfs-progs: Preserve process_one_leaf return value.

btrfs-progs: Preserve process_one_leaf return value.

The return value in process_one_leaf could be over-written while
looping over the items in the leaf.

This patch will preserve a non-zero return value to the calling
function if a non-zero return value is encountered in the loop.

The return value of one (1) is consistent with non-zero values
that could be returned while processing the leaf.

The only caller of this function (walk_down_tree) would ignore
the return value anyway.  But this patch will correct the
behaviour in case future changes intend to utilize the return
value.

Signed-off-by: Mitch Harder <mitch.harder@sabayonlinux.org>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>