match sendfile() error handling to send().

Sendfile() should match the error checking order of send() which
is currently:

SBS_CANTSENDMORE
so_error
SS_ISCONNECTED

Submitted by: Jason Eggleston <jason@eggnet.com>
Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12633