fs: add hole punching to fallocate

Hole punching has already been implemented by XFS and OCFS2, and has the
potential to be implemented on both BTRFS and EXT4 so we need a generic way to
get to this feature.  The simplest way in my mind is to add FALLOC_FL_PUNCH_HOLE
to fallocate() since it already looks like the normal fallocate() operation.
I've tested this patch with XFS and BTRFS to make sure XFS did what it's
supposed to do and that BTRFS failed like it was supposed to.  Thank you,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -223,7 +223,12 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 		return -EINVAL;
 
 	/* Return error if mode is not supported */
-	if (mode && !(mode & FALLOC_FL_KEEP_SIZE))
+	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
+		return -EOPNOTSUPP;
+
+	/* Punch hole must have keep size set */
+	if ((mode & FALLOC_FL_PUNCH_HOLE) &&
+	    !(mode & FALLOC_FL_KEEP_SIZE))
 		return -EOPNOTSUPP;
 
 	if (!(file->f_mode & FMODE_WRITE))

include/linux/falloc.h

@@ -2,6 +2,7 @@
 #define _FALLOC_H_
 
 #define FALLOC_FL_KEEP_SIZE	0x01 /* default is extend size */
+#define FALLOC_FL_PUNCH_HOLE	0x02 /* de-allocates range */
 
 #ifdef __KERNEL__