ext4/305: reduce runtime by limiting mount/umount cycles

ext4/305: reduce runtime by limiting mount/umount cycles

ext4/305 sleeps 3 minutes and does mount/umount loop in background,
which produces lots of logs in dmesg and 3 minutes is not necessary.

Ted pointed out that 30 mount/umount cycles is enough to crash a buggy
kernel, so just limit the mount/umount loop to reduce the runtime. And
now the runtime is about 2s.

Reported-by: Theodore Ts'o <tytso@mit.edu>
Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>