splice: fix infinite loop in generic_file_splice_read()

There's a quirky loop in generic_file_splice_read() that could go
on indefinitely, if the file splice returns 0 permanently (and not
just as a temporary condition). Get rid of the loop and pass
back -EAGAIN correctly from __generic_file_splice_read(), so we
handle that condition properly as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>