Use SEEK_{SET,CUR,END} for file seek "whence"

Use either SEEK_* or 0,1,2..., but not both.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8656