nfscl: Fix use after free for forced dismount

When a forced dismount is done and delegations are being
issued by the server (disabled by default for FreeBSD
servers), the delegation structure is free'd before the
loop calling vflush(). This could result in a use after
free of the delegation structure.

This patch changes the code so that the delegation
structures are not free'd until after the vflush()
loop for forced dismounts.

Found during a recent IETF NFSv4 working group testing event.

MFC after: 2 weeks