libblkid: (xfs) external log: check for regular xfs on more sectors

libblkid: (xfs) external log: check for regular xfs on more sectors

The xfs external log probe only checks for regular xfs on sector zero,
but then checks for valid log record headers on all first 512 sectors.

This can incorrectly detect an xfs external log if a regular xfs (i.e.
with internal log) is shifted by up to 512 sectors; it may happen with
bcache and LVM1 for example, as the regular xfs is found later in disk.

This results in ambivalent filesystem detection, thus no UUID for udev.

Fix this problem by checking for regular xfs on all sectors considered
by the xfs external log probe.

Test-case with bcache:
---

    $ IMG=bcache-backing-device.img
    $ dd if=/dev/zero of=$IMG bs=1G count=0 seek=1
    $ DEV=$(sudo losetup --find --show $IMG)

    $ sudo make-bcache -B $DEV

    $ sudo mkfs.xfs -d agsize=16m -l agnum=0 -f /dev/bcache0

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs /dev/bcache0
    DEVICE  OFFSET TYPE UUID                                 LABEL
    bcache0 0x0    xfs  9f6dfa9d-4488-46f7-906b-dcfc96027cfe

    $ echo 1 | sudo tee /sys/block/bcache0/bcache/stop

    $ sudo hexdump -C $DEV | grep -m2 -e XFSB -e 'fe ed ba be'
    00002000  58 46 53 42 00 00 10 00  00 00 00 00 00 03 f0 00  |XFSB............|
    00007000  fe ed ba be 00 00 00 01  00 00 00 02 00 00 00 14  |................|

  Without patch:

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs $DEV
    DEVICE OFFSET TYPE             UUID                                 LABEL
    loop0  0x1018 bcache           23da3ba9-2467-453d-b020-06f02c947190
    loop0  0x7000 xfs_external_log

  With patch:

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs $DEV
    DEVICE OFFSET TYPE             UUID                                 LABEL
    loop0  0x1018 bcache           23da3ba9-2467-453d-b020-06f02c947190

Test-case with LVM1:
---

    $ IMG=lvm-backing-device.img
    $ dd if=/dev/zero of=$IMG bs=1G count=0 seek=1
    $ DEV=$(sudo losetup --find --show $IMG)

    $ sudo lvm pvcreate -M1 $DEV
    $ sudo lvm vgcreate -M1 lvm-vg-test $DEV
    $ sudo lvm lvcreate  lvm-vg-test --name lvm-lv-test --extents 100%VG

    $ sudo mkfs.xfs -d agsize=16m -l agnum=0 -f /dev/mapper/lvm--vg--test-lvm--lv--test

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs /dev/mapper/lvm--vg--test-lvm--lv--test
    DEVICE                      OFFSET TYPE UUID                                 LABEL
    lvm--vg--test-lvm--lv--test 0x0    xfs  451ba725-8394-4ebe-9b49-fc5f4a99667f

    $ sudo lvchange -an lvm-vg-test

    $ sudo hexdump -C $DEV | grep -m2 -e XFSB -e 'fe ed ba be'
    00020000  58 46 53 42 00 00 10 00  00 00 00 00 00 03 f0 00  |XFSB............|
    00025000  fe ed ba be 00 00 00 01  00 00 00 02 00 00 00 14  |................|

  Without patch:

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs $DEV
    DEVICE OFFSET  TYPE             UUID                                   LABEL
    loop0  0x0     LVM1_member      agUhNT-9f42-Z30B-Z4Ew-skWd-3h3a-tWMY0A
    loop0  0x25000 xfs_external_log

  With patch:

    $ sudo LD_LIBRARY_PATH=./.libs ./wipefs $DEV
    DEVICE OFFSET TYPE        UUID                                   LABEL
    loop0  0x0    LVM1_member agUhNT-9f42-Z30B-Z4Ew-skWd-3h3a-tWMY0A

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>