wipefs does not properly clean up vfat signatures

With the current version you need to call wipefs three times on vfat before it succeeds. This is reproduced with the udisks2 test suite, and with these manual commands:

$ dd if=/dev/zero of=/tmp/img bs=1M count=50 && mkfs.vfat -n txx /tmp/img && wipefs -a /tmp/img && blkid -p /tmp/img
8 bytes were erased at offset 0x36 (vfat)
they were: 46 41 54 31 36 20 20 20
/tmp/img: SEC_TYPE="msdos" LABEL="txx" UUID="3F05-CCD3" VERSION="FAT16" TYPE="vfat" USAGE="filesystem"

After second time:
$ wipefs -a /tmp/img && blkid -p /tmp/img
1 bytes were erased at offset 0x0 (vfat)
they were: eb
/tmp/img: SEC_TYPE="msdos" LABEL="txx" UUID="3F05-CCD3" VERSION="FAT16" TYPE="vfat" USAGE="filesystem"

Third time is the charm:
$ wipefs -a /tmp/img && blkid -p /tmp/img
2 bytes were erased at offset 0x1fe (vfat)
they were: 55 aa

This is fixed in the current upstream version (see bug 1012081), but I'd like to fix it for Quantal as this makes other test suite regressions in udisks hard to see (we missed a regression on i386 for weeks due to this).