devicemapper: remove 256 character limit of libdm logs

This limit is unecessary and can lead to the truncation of long libdm
logs (which is quite annoying).

Fixes: b440ec0 ("device-mapper: Move all devicemapper spew to log through utils.Debugf().")
Signed-off-by: Aleksa Sarai <asarai@suse.de>

pkg/devicemapper/devmapper_wrapper.go

@@ -4,6 +4,7 @@ package devicemapper
 
 /*
 #cgo LDFLAGS: -L. -ldevmapper
+#define _GNU_SOURCE
 #include <libdevmapper.h>
 #include <linux/fs.h>   // FIXME: present only for BLKGETSIZE64, maybe we can remove it?
 
@@ -12,19 +13,20 @@ extern void DevmapperLogCallback(int level, char *file, int line, int dm_errno_o
 
 static void	log_cb(int level, const char *file, int line, int dm_errno_or_class, const char *f, ...)
 {
-  char buffer[256];
-  va_list ap;
+	char *buffer = NULL;
+	va_list ap;
 
-  va_start(ap, f);
-  vsnprintf(buffer, 256, f, ap);
-  va_end(ap);
+	va_start(ap, f);
+	vasprintf(&buffer, f, ap);
+	va_end(ap);
 
-  DevmapperLogCallback(level, (char *)file, line, dm_errno_or_class, buffer);
+	DevmapperLogCallback(level, (char *)file, line, dm_errno_or_class, buffer);
+	free(buffer);
 }
 
 static void	log_with_errno_init()
 {
-  dm_log_with_errno_init(log_cb);
+	dm_log_with_errno_init(log_cb);
 }
 */
 import "C"