Linux 4.8 compat

[behlendorf]

New kernel release window, new build failures.

spl/spl/spl-rwlock.c: In function ‘__rwsem_tryupgrade’:
spl/spl/spl-rwlock.c:55:2: error: invalid initializer
  val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
  ^
spl/spl/spl-rwlock.c:55:2: error: invalid initializer
spl/spl/spl-rwlock.c:57:14: error: invalid operands to binary == (have ‘atomic_long_t’ and ‘long int’)
  return (val == SPL_RWSEM_SINGLE_READER_VALUE);
              ^
spl/spl/spl-rwlock.c:58:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
  CC      mm/rmap.o
make[2]: *** [spl/spl/spl-rwlock.o] Error 1
make[1]: *** [spl/spl] Error 2
make: *** [spl] Error 2
make: *** Waiting for unfinished jobs....