btrfs-progs: fix improper return value check for is_existing_blk_or_reg_file

btrfs-progs: fix improper return value check for is_existing_blk_or_reg_file

The function @is_existing_blk_or_reg_file has a return value of -errno,
which indicate the @stat call fails with non-ENOENT errors.
In this condition, we should not continue the following work.

But -errno evaluates to true and will let the following work go.
So we should judge more accurately whether the return value of
@is_existing_blk_or_reg_file is > 0 or not to decide our behavior.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>