Ignore pre-read for character devices

Ignore pre-read for character devices

pre_read_file() could ignore a chrdev due to its purpose of providing
unbuffered access to devices not limited to seekable disk devices,
while the purpose of this function (i.e. pre_read= option) is to
lseek(2) the given offset and page cache whatever read via read(2)
which is basically for filesystems and blkdevs.

(This commit directly goes on top of the one before previous one)

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