bug#60291: chmod a-r does not fail for vfat

[Krzysztof Żelechowski]

chmod (GNU coreutils) 9.1 x86_64

cd /tmp
dd if=/dev/zero of=fd0 count=2880
echo 'drive a:
        file="/tmp/fd0' >.mtoolsrc
mformat a:
umask 0022
udisksctl loop-setup -f fd0
udisks mount /dev/loop3p1
cd /run/media/dell/1F9D-2F6C
echo >a.txt
chmod a-r a.txt && stat a.txt

Expected: chmod: unsupported permissions
Got: -r--r--r--
This is wrong because the command chmod had failed to set the permissions as 
required but it had claimed to have succeeded.

Similarly, chmod a-w and chmod o+w should succeed but chmod a+w should fail 
(because the result is o+w).