mpage: mpage_readpages() should submit IO as read-ahead

mpage: mpage_readpages() should submit IO as read-ahead

a_ops->readpages() is only ever used for read-ahead, yet we don't
flag the IO being submitted as such. Fix that up. Any file system
that uses mpage_readpages() as its ->readpages() implementation
will now get this right.

Since we're passing in whether the IO is read-ahead or not, we
don't need to pass in the 'gfp' separately, as it is dependent
on the IO being read-ahead. Kill off that member.

Add some documentation notes on ->readpages() being purely for
read-ahead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>