locking: Add rwsem_assert_held() and rwsem_assert_held_write()

Modelled after lockdep_assert_held() and lockdep_assert_held_write(),
but are always active, even when lockdep is disabled.  Of course, they
don't test that _this_ thread is the owner, but it's sufficient to catch
many bugs and doesn't incur the same performance penalty as lockdep.

Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>