switch file_open_root() to struct path

... and provide file_open_root_mnt(), using the root of given mount.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 0302035..9bb2b35 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -890,3 +890,12 @@
 
 mnt_want_write_file() can now only be paired with mnt_drop_write_file(),
 whereas previously it could be paired with mnt_drop_write() as well.
+
+---
+
+**mandatory**
+
+Calling conventions for file_open_root() changed; now it takes struct path *
+instead of passing mount and dentry separately.  For callers that used to
+pass <mnt, mnt->mnt_root> pair (i.e. the root of given mount), a new helper
+is provided - file_open_root_mnt().  In-tree users adjusted.