mm: remove page_mapping()

mm: remove page_mapping()

All callers are now converted, delete this compatibility wrapper.  Also
fix up some comments which referred to page_mapping.

Link: https://lkml.kernel.org/r/20240423225552.4113447-7-willy@infradead.org
Link: https://lkml.kernel.org/r/20240524181813.698813-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>