btrfs-progs: Fix wrong return value when executing 'fi show' on umounted device.

btrfs-progs: Fix wrong return value when executing 'fi show' on umounted device.

When executing 'btrfs fi show' on unmounted device, even no problem
happens, the return value is still 1 not 0.

The problem lies in search_umounted_fs_uuids(), where when it finds the
given uuid, it should return 1, but later uuid copy overwrites the return
value, causing it always return 0 under that case.

Fix it by pass found as pointer, and return value only indicates
whether anything wrong happens, whether found or not is stored in the
new parameter.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>