mm/doc: turn fault flags into an enum

The kernel-doc script complains about

  include/linux/mm.h:425: warning: wrong kernel-doc identifier on line:
   * Fault flag definitions.

I don't know how to document a series of #defines, so turn these
definitions into an enum and document that instead.

Link: https://lkml.kernel.org/r/20210322195022.2143603-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>