[klibc] loop: switch to linux/loop.h

linux/loop.h header is exported by linux, for userspace to
consume. This would prevent issues with struct sizes
incompatibilities.

References: https://bugs.launchpad.net/ubuntu/+source/klibc/+bug/1850184
Signed-off-by: Michael Hudson-Doyle <michael.hudson@ubuntu.com>
Signed-off-by: Adam Conrad <adconrad@ubuntu.com>
Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
[bwh: We were using dev_t instead of __kernel_old_dev_t, which doesn't
 match on some architectures.  Instead of fixing our definition first,
 switch directly to the kernel's UAPI header.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>