btrfs-progs: fuzz-test: Add test case for unaligned extent item

Reported-by: Lukas Lueg <lukas.lueg@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ added bko-NNN- prefix to the files ]
Signed-off-by: David Sterba <dsterba@suse.com>

tests/fuzz-tests/images/bko-155181-unaligned-extent-item.raw.txt

@@ -0,0 +1,8 @@
+URL: https://bugzilla.kernel.org/show_bug.cgi?id=155181
+Lukas Lueg 2016-08-28 10:52:32 UTC
+
+Created attachment 230891 [details]
+BTRFS-image that reaches abort() in btrfsck
+
+More news from the fuzzer. The attached image causes btrfsck to reach abort()
+in in cmds-check.c:add_tree_backref(); using btrfs-progs v4.7-42-g56e9586.