Handle partial data re-sending on ktls/sendfile on FreeBSD

Add a handler for EBUSY sendfile error in addition to
EAGAIN. With EBUSY returned the data still can be partially
sent and user code has to be notified about it, otherwise it
may try to send data multiple times.

PR: 251969
Reviewed by: jkim
Obtained from: OpenSSL (dfcfd17f2818cf520ce6381aed9ec3d2fc12170d)
MFC after: 1 week
Sponsored by: Netflix (merging to FreeBSD)
Differential Revision: https://reviews.freebsd.org/D28714