CIFS: Add direct I/O functions to file_operations

With direct read/write functions implemented, add them to file_operations.

Dircet I/O is used under two conditions:
1. When mounting with "cache=none", CIFS uses direct I/O for all user file
data transfer.
2. When opening a file with O_DIRECT, CIFS uses direct I/O for all data
transfer on this file.

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>

fs/cifs/cifsfs.c

@@ -1180,9 +1180,8 @@ const struct file_operations cifs_file_strict_ops = {
 };
 
 const struct file_operations cifs_file_direct_ops = {
-	/* BB reevaluate whether they can be done with directio, no cache */
-	.read_iter = cifs_user_readv,
-	.write_iter = cifs_user_writev,
+	.read_iter = cifs_direct_readv,
+	.write_iter = cifs_direct_writev,
 	.open = cifs_open,
 	.release = cifs_close,
 	.lock = cifs_lock,
@@ -1236,9 +1235,8 @@ const struct file_operations cifs_file_strict_nobrl_ops = {
 };
 
 const struct file_operations cifs_file_direct_nobrl_ops = {
-	/* BB reevaluate whether they can be done with directio, no cache */
-	.read_iter = cifs_user_readv,
-	.write_iter = cifs_user_writev,
+	.read_iter = cifs_direct_readv,
+	.write_iter = cifs_direct_writev,
 	.open = cifs_open,
 	.release = cifs_close,
 	.fsync = cifs_fsync,