cifs: fix fallocate when trying to allocate a hole.

Remove the conditional checking for out_data_len and skipping the fallocate
if it is 0. This is wrong will actually change any legitimate the fallocate
where the entire region is unallocated into a no-op.

Additionally, before allocating the range, if FALLOC_FL_KEEP_SIZE is set then
we need to clamp the length of the fallocate region as to not extend the size of the file.

Fixes: 966a3cb7 ("cifs: improve fallocate emulation")
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -3667,11 +3667,6 @@ static int smb3_simple_fallocate_range(unsigned int xid,
 			(char **)&out_data, &out_data_len);
 	if (rc)
 		goto out;
-	/*
-	 * It is already all allocated
-	 */
-	if (out_data_len == 0)
-		goto out;
 
 	buf = kzalloc(1024 * 1024, GFP_KERNEL);
 	if (buf == NULL) {
@@ -3794,6 +3789,24 @@ static long smb3_simple_falloc(struct file *file, struct cifs_tcon *tcon,
 		goto out;
 	}
 
+	if (keep_size == true) {
+		/*
+		 * We can not preallocate pages beyond the end of the file
+		 * in SMB2
+		 */
+		if (off >= i_size_read(inode)) {
+			rc = 0;
+			goto out;
+		}
+		/*
+		 * For fallocates that are partially beyond the end of file,
+		 * clamp len so we only fallocate up to the end of file.
+		 */
+		if (off + len > i_size_read(inode)) {
+			len = i_size_read(inode) - off;
+		}
+	}
+
 	if ((keep_size == true) || (i_size_read(inode) >= off + len)) {
 		/*
 		 * At this point, we are trying to fallocate an internal