userfaultfd: Rename uffd_api.bits into .features

userfaultfd: Rename uffd_api.bits into .features

This is (seems to be) the minimal thing that is required to unblock
standard uffd usage from the non-cooperative one.  Now more bits can be
added to the features field indicating e.g.  UFFD_FEATURE_FORK and others
needed for the latter use-case.

Signed-off-by: Pavel Emelyanov <[email protected]>
Signed-off-by: Andrea Arcangeli <[email protected]>
Cc: Sanidhya Kashyap <[email protected]>
Cc: [email protected]
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Andres Lagar-Cavilla <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Peter Feiner <[email protected]>
Cc: "Dr. David Alan Gilbert" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Huangpeng (Peter)" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>