[IB] umad: further ib_unregister_mad_agent() deadlock fixes

The previous umad deadlock fix left ib_umad_kill_port() still
vulnerable to deadlocking.  This patch fixes that by downgrading our
lock to a read lock when we might end up trying to reacquire the lock
for reading.

Signed-off-by: Roland Dreier <rolandd@cisco.com>