mkfs: fix blkid probe API violations causing weird output

mkfs: fix blkid probe API violations causing weird output

The blkid_do_fullprobe function in libblkid 2.38.1 will try to read the
last 512 bytes off the end of a block device.  If the block device has a
2k LBA size, that read will fail.  blkid_do_fullprobe passes the -EIO
back to the caller (mkfs) even though the API documentation says it
only returns 1, 0, or -1.

Change the "cannot detect existing fs" logic to look for any negative
number.  Otherwise, you get unhelpful output like this:

$ mkfs.xfs -l size=32m -b size=4096 /dev/loop3
mkfs.xfs: Use the -f option to force overwrite.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>