Documentation/filesystems: fix documentation for ->getattr()

Following the recent merge of statx, correct the documented prototype
for the ->getattr() inode operation, and add an entry to the porting
file.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 9528007..5fb17f4 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -600,3 +600,9 @@
 [recommended]
 	->readlink is optional for symlinks.  Don't set, unless filesystem needs
 	to fake something for readlink(2).
+--
+[mandatory]
+	->getattr() is now passed a struct path rather than a vfsmount and
+	dentry separately, and it now has request_mask and query_flags arguments
+	to specify the fields and sync type requested by statx.  Filesystems not
+	supporting any statx-specific features may ignore the new arguments.