[XFS] Unwrap mru_lock.

Un-obfuscate mru_lock, remove mutex_lock->spin_lock macros, call spin_lock
directly, remove extraneous cookie holdover from old xfs code.

SGI-PV: 970382
SGI-Modid: xfs-linux-melb:xfs-kern:29745a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>