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.

include/crucible/fd.h

@@ -13,6 +13,10 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+// ioctl
+#include <sys/ioctl.h>
+#include <linux/fs.h>
+
 // socket
 #include <sys/socket.h>
 
@@ -141,6 +145,8 @@ namespace crucible {
 		Stat &lstat(const string &filename);
 	};
 
+	int ioctl_iflags_get(int fd);
+
 	string st_mode_ntoa(mode_t mode);
 
 	// Because it's not trivial to do correctly