[IA64] Fix rwsem: RWSEM_WAITING_BIAS must not be unsigned.

Some nice improvements were made to rwsem in commit:

 424acaaeb3a3932d64a9b4bd59df6cf72c22d8f3
 rwsem: wake queued readers when writer blocks on active read lock

but this change overlooked that ia64 had defined RWSEM_WAITING_BIAS
as an unsigned value, while the new code required a signed value (as
it is in every other architecture).

This fix suggested by the original patch author: Michel Lespinasse.

Signed-off-by: Tony Luck <tony.luck@intel.com>