Check so_error early in sendfile() call. Prior to this patch, if a
connection was reset by the remote end, sendfile() would just report
ENOTCONN instead of ECONNRESET.

Submitted by: Jason Eggleston <jason@eggnet.com>
Reviewed by: glebius
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12575