generic/421: stop test from hanging on buggy kernels

generic/421: stop test from hanging on buggy kernels

On some buggy kernels, the 'find' command in generic/421 encounters
a "Permission denied" error when trying to search $dir, so it
doesn't find the file it's supposed to.  This causes 'cat' to read
from stdin, hanging the test.  Quote the argument to cat to make the
test fail right away rather than hanging.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>