fs: prevent doing FALLOC_FL_ZERO_RANGE on append only file

Currently punch hole and collapse range fallocate operation are not
allowed on append only file. This should be case for zero range as well.
Fix it by allowing only pure fallocate (possibly with keep size set).
Signed-off-by: NLukas Czerner <lczerner@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/open.c

@@ -254,11 +254,9 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 		return -EBADF;
 
 	/*
-	 * It's not possible to punch hole or perform collapse range
-	 * on append only file
+	 * We can only allow pure fallocate on append only files
 	 */
-	if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_COLLAPSE_RANGE)
-	    && IS_APPEND(inode))
+	if ((mode & ~FALLOC_FL_KEEP_SIZE) && IS_APPEND(inode))
 		return -EPERM;
 
 	if (IS_IMMUTABLE(inode))