Linux 3.2 compat: rw_semaphore.wait_lock is raw

Linux 3.2 compat: rw_semaphore.wait_lock is raw

The wait_lock member of the rw_semaphore struct became a raw_spinlock_t
in Linux 3.2 at torvalds/linux@ddb6c9b58a19edcfac93ac670b066c836ff729f1.

Wrap spin_lock_* function calls in a new spl_rwsem_* interface to
ensure type safety if raw_spinlock_t becomes architecture specific,
and to satisfy these compiler warnings:

  warning: passing argument 1 of ‘spinlock_check’
    from incompatible pointer type [enabled by default]
  note: expected ‘struct spinlock_t *’
    but argument is of type ‘struct raw_spinlock_t *’

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #76
Closes: zfsonlinux/zfs#463