Skip nocow files to speed up processing

If you have a lot of or a few big nocow files (like vm images) which
contain a lot of potential deduplication candidates, bees becomes
incredibly slow running through a lot "invalid operation" exceptions.

Let's just skip over such files to get more bang for the buck. I did no
regression testing as this patch seems trivial (and I cannot imagine any
pitfalls either). The process progresses much faster for me now.

lib/fd.cc

@@ -488,6 +488,14 @@ namespace crucible {
 		lstat(filename);
 	}
 
+	int
+	ioctl_iflags_get(int fd)
+	{
+		int attr = 0;
+		DIE_IF_MINUS_ONE(ioctl(fd, FS_IOC_GETFLAGS, &attr));
+		return attr;
+	}
+
 	string
 	readlink_or_die(const string &path)
 	{