kern_sendfile.c: do not release sfio reference on error.

It is already done by sendfile_iodone(), now consistently for all errors.
This de-facto reverts r358597, after r359466.

Reviewed by: glebius, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24038