dissect: split out DM deferred remove into src/shared/dm-util.c

The function is generally useful, let's split it out so that we can make
use of it later on in systemd-homed.

src/shared/dm-util.c

@@ -0,0 +1,41 @@
+#include <fcntl.h>
+#include <linux/dm-ioctl.h>
+#include <string.h>
+#include <sys/ioctl.h>
+
+#include "dm-util.h"
+#include "fd-util.h"
+
+int dm_deferred_remove(const char *name) {
+
+        struct dm_ioctl dm = {
+                .version = {
+                        DM_VERSION_MAJOR,
+                        DM_VERSION_MINOR,
+                        DM_VERSION_PATCHLEVEL
+                },
+                .data_size = sizeof(dm),
+                .flags = DM_DEFERRED_REMOVE,
+        };
+
+        _cleanup_close_ int fd = -1;
+
+        assert(name);
+
+        /* Unfortunately, libcryptsetup doesn't provide a proper API for this, hence call the ioctl()
+         * directly. */
+
+        if (strlen(name) > sizeof(dm.name)-1)
+                return -ENODEV; /* A device with a name longer than this cannot possibly exist */
+
+        fd = open("/dev/mapper/control", O_RDWR|O_CLOEXEC);
+        if (fd < 0)
+                return -errno;
+
+        strncpy(dm.name, name, sizeof(dm.name));
+
+        if (ioctl(fd, DM_DEV_REMOVE, &dm))
+                return -errno;
+
+        return 0;
+}