um: Fix __swp_type()

commit 2b76ebaa728f8a3967c52aa189261c72fe56a6f1 upstream.

The current __swp_type() function uses a too small bitshift.
Using more than one swap files causes bad pages because
the type bits clash with other page flags.

Analyzed-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>