libzpool: Fix ->rw_count type in struct krwlock

Fixes the following compilation error:

  ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.c: In function ‘rw_enter’:
  ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.c:205:3: error: pointer targets in passing argument 1 of ‘atomic_add_int’ differ in signedness [-Werror=pointer-sign]
     atomic_add_int(&rwlp->rw_count, 1);
     ^
  In file included from ../../bsd/sys/cddl/compat/opensolaris/sys/atomic.h:33:0,
                   from ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/sys/zfs_context.h:66,
                   from ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/spa.h:31,
                   from ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.c:32:
  ../../bsd/machine/atomic.h:285:1: note: expected ‘volatile u_int *’ but argument is of type ‘int *’
   ATOMIC_ASM(add,      int,   "addl %1,%0",  "ir",  v);
   ^

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>