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>