msi: Remove msi_lock.

With the removal of msi_lookup_irq all of the functions using msi_lock
operated on a single device and none of them could reasonably be
called on that device at the same time. 

Since what little synchronization that needs to happen needs to happen
outside of the msi functions, msi_lock could never be contended and as
such is useless and just complicates the code.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>