btrfs-progs: Fix the return value of btrfs_scan_kernel()

btrfs-progs: Fix the return value of btrfs_scan_kernel()

btrfs_scan_kernel() is only used in 'btrfs fi show' but returns wrong
return value.  When search parameter is passed, it will never return 0
even the search can be matched.

This patch will change the whatever strange logic to a more easy to
understand one using 'found' var.

Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Cc: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>