drop logging when blkdev invalidation failed on unsupported platforms

22de5d77('Use ENOTSUP if OS doesn't support blkdev page cache invalidation')
changed errno values being used on supported platforms to ENOTSUP,
and also log_info'd it via strerror(), but strerror() isn't working
on some platform and instead prints irrelevant string.

Re: fio Windows Invalidate
http://www.spinics.net/lists/fio/msg06084.html

The logging of ENOTSUP could just be dropped since this isn't a real
failure, as many platforms either don't have the idea of invalidating
blkdev's page cache mapping, or use chrdev for block devices.

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