loopback: separate loop logic from devicemapper

The loopback logic is not technically exclusive to the devicemapper
driver. This reorganizes the code such that the loopback code is usable
outside of the devicemapper package and driver.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

loopback/loop_wrapper.go

@@ -0,0 +1,52 @@
+// +build linux
+
+package loopback
+
+/*
+#include <linux/loop.h> // FIXME: present only for defines, maybe we can remove it?
+
+#ifndef LOOP_CTL_GET_FREE
+  #define LOOP_CTL_GET_FREE 0x4C82
+#endif
+
+#ifndef LO_FLAGS_PARTSCAN
+  #define LO_FLAGS_PARTSCAN 8
+#endif
+
+*/
+import "C"
+
+type loopInfo64 struct {
+	loDevice         uint64 /* ioctl r/o */
+	loInode          uint64 /* ioctl r/o */
+	loRdevice        uint64 /* ioctl r/o */
+	loOffset         uint64
+	loSizelimit      uint64 /* bytes, 0 == max available */
+	loNumber         uint32 /* ioctl r/o */
+	loEncryptType    uint32
+	loEncryptKeySize uint32 /* ioctl w/o */
+	loFlags          uint32 /* ioctl r/o */
+	loFileName       [LoNameSize]uint8
+	loCryptName      [LoNameSize]uint8
+	loEncryptKey     [LoKeySize]uint8 /* ioctl w/o */
+	loInit           [2]uint64
+}
+
+// IOCTL consts
+const (
+	LoopSetFd       = C.LOOP_SET_FD
+	LoopCtlGetFree  = C.LOOP_CTL_GET_FREE
+	LoopGetStatus64 = C.LOOP_GET_STATUS64
+	LoopSetStatus64 = C.LOOP_SET_STATUS64
+	LoopClrFd       = C.LOOP_CLR_FD
+	LoopSetCapacity = C.LOOP_SET_CAPACITY
+)
+
+// LOOP consts.
+const (
+	LoFlagsAutoClear = C.LO_FLAGS_AUTOCLEAR
+	LoFlagsReadOnly  = C.LO_FLAGS_READ_ONLY
+	LoFlagsPartScan  = C.LO_FLAGS_PARTSCAN
+	LoKeySize        = C.LO_KEY_SIZE
+	LoNameSize       = C.LO_NAME_SIZE
+)