btrfs-progs: Use long type to get thread's return value

btrfs-progs: Use long type to get thread's return value

pthread use void * to save return status, we can use this pointer to
save our return value, but we need to keep the same length.

This patch move to use long type variable to save return value
of our thread, to avoid potential invalid memory access.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>