mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()

mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()

folio_is_secretmem() is currently only used during GUP-fast.  Nowadays,
folio_fast_pin_allowed() performs similar checks during GUP-fast and
contains a lot of careful handling -- READ_ONCE() -- , sanity checks --
lockdep_assert_irqs_disabled() -- and helpful comments on how this
handling is safe and correct.

So let's merge folio_is_secretmem() into folio_fast_pin_allowed().  Rename
folio_fast_pin_allowed() to gup_fast_folio_allowed(), to better match the
new semantics.

Link: https://lkml.kernel.org/r/20240326143210.291116-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: xingwei lee <xrivendell7@gmail.com>
Cc: yue sun <samsun1006219@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>