rwsem: make the waiter type an enumeration rather than a bitmask

this is patch 1/12: https://lkml.org/lkml/2013/3/6/566



found this patch series commited by faux123.

We are not planning to add some new waiter flags, so we can convert the
waiter type into an enumeration.

Background: David Howells suggested I do this back when I tried adding
a new waiter type for unfair readers. However, I believe the cleanup
applies regardless of that use case.

Change-Id: I45669f17017a82f666af3be9392179e567f462fa
Signed-off-by: Michel Lespinasse <walken@google.com>