Delete invalid assignment statements in do_sendfile

Assigning value -EINVAL to "retval" here, but that stored value is
overwritten before it can be used.

retval = -EINVAL;
....
retval = rw_verify_area(WRITE, out.file, &out_pos, count);

value_overwrite: Overwriting previous write to "retval" with value
from rw_verify_area

delete invalid assignment statements

Signed-off-by: n00202754 <nixiaoming@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>