add ifdef __sun__ for Solaris specific code

add ifdef __sun__ for Solaris specific code

ENOTTY on direct I/O setup failure via directio(3C) is Solaris (ZFS)
specific (as also mentioned in ZoL commit[1]).

Linux and many others which support O_DIRECT never come here, but
others supporting direct I/O via OS specific way don't necessarily
have the same ENOTTY behavior or ZFS itself.

This commit brings back the original log before 42f1ee68 for others.

[1] https://github.com/zfsonlinux/zfs/commit/a08c76a8ad63c28384ead72b53a3d7ef73f39357

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>