userfaultfd: convert to use anon_inode_getfd()

Nothing actually calls userfaultfd_file_create() besides the
userfaultfd() system call itself.  So simplify things by folding it into
the system call and using anon_inode_getfd() instead of
anon_inode_getfile().  Do the same in resolve_userfault_fork() as well.

This removes over 50 lines with no change in functionality.

Link: http://lkml.kernel.org/r/20171229212403.22800-1-ebiggers3@gmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>