btrfs-progs: fsck-test: add a test case for unexpected file extent items

The image is created using btrfs-corrupt-block, the fs tree leaf looks
like this:

	item 0 key (256 INODE_ITEM 0) itemoff 16123 itemsize 160
		generation 3 transid 9 size 12 nbytes 16384
		block group 0 mode 40755 links 1 uid 0 gid 0 rdev 0
		sequence 1 flags 0x0(none)
	item 1 key (256 INODE_REF 256) itemoff 16111 itemsize 12
		index 0 namelen 2 name: ..
	item 2 key (256 DIR_ITEM 496027801) itemoff 16075 itemsize 36
		location key (257 INODE_ITEM 0) type BLKDEV <<<
		transid 9 data_len 0 name_len 6
		name: foobar
	item 3 key (256 DIR_INDEX 2) itemoff 16039 itemsize 36
		location key (257 INODE_ITEM 0) type BLKDEV <<<
		transid 9 data_len 0 name_len 6
		name: foobar
	item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
		generation 9 transid 9 size 8192 nbytes 8192
		block group 0 mode 60660 links 1 uid 0 gid 0 rdev 0 <<<
		sequence 2 flags 0x0(none)
	item 5 key (257 INODE_REF 256) itemoff 15863 itemsize 16
		index 2 namelen 6 name: foobar
	item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
		generation 9 type 1 (regular)
		extent data disk byte 13631488 nr 8192
		extent data offset 0 nr 8192 ram 8192
		extent compression 0 (none)

The image is generated by the following steps:

 # mkfs.btrfs -f $dev
 # mount $dev $mnt
 # xfs_io -f -c "pwrite 0 4k" $mnt/foobar
 # umount $mnt

 ## Change the inode's mode from REG to BLK
 # btrfs-corrupt-block -i 257 -f mode --value 25008 $dev

 ## Change the dir item's flag from REG to BLK
 # btrfs-corrupt-block -D 256,84,496027801 -f flags --value 4 $dev

 ## Change the dir index's flag from REG to BLK
 # btrfs-corrupt-block -D 256,96,2 -f flags --value 4 $dev

The inode's mode and dir flags all match, but there is one unexpected
file extent item there.

Both lowmem and original mode should detect such problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

tests/fsck-tests/075-blk-inode-with-data/default.img.xz

@@ -0,0 +1,16 @@
+#!/bin/bash
+#
+# Verify that check can detect unexpected file extent items for special inodes
+# which should not have any file extent.
+
+source "$TEST_TOP/common" || exit
+
+check_prereq btrfs
+
+check_image() {
+	# The image contains an BLK inode, with a file extent.
+	# The check should report an error.
+	run_mustfail "unexpected file extent not detected" "$TOP/btrfs" check "$1"
+}
+
+check_all_images