loop: preallocate eight loop devices

The kernel on-demand loop device instantiation breaks several user space
tools as the tools are not ready to cope with the "on-demand feature".  Fix
it by instantiate default 8 loop devices and also reinstate max_loop module
parameter.

Signed-off-by: Ken Chen <kenchen@google.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>