sendfile: Explicitly ignore errors from copyout()

There is a documented bug in sendfile.2 which notes that sendfile(2)
does not raise an error if it fails to copy out the number of bytes
written. Explicitly ignore the error from copyout() calls in
preparation for annotating copyout() with __result_use_check.

Reviewed by: glebius, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43129