btrfs-progs: fix wrong error msg for exec btrfsck as non-root

btrfs-progs: fix wrong error msg for exec btrfsck as non-root

When exec btrfsck as non-root user on a disk, btrfsck will always
warn that "No such file or directory", despite that a directory
(e.g. /dev/vboxusb)actually exists. We just have no permission.
In this case, return the -errno set by the opendir call in
btrfs_scan_one_dir rather than blindly return -ENOENT.

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