Linux 3.2 compat: rw_semaphore.wait_lock is raw

[dajhorn]

The wait_lock member of the rw_semaphore struct became a raw_spinlock_t
in Linux 3.2 at torvalds/linux@ddb6c9b.

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 *’

Closes: #76
Closes: openzfs/zfs#463

[behlendorf]

Exactly what I had in mind. Thanks, this was merged as commit 588d900