MFC r310092:

Add a free_iovec() function to reset iovec's.

The primary purpose is to call nmount() in a loop with new iovec's so
free_iovec takes arguments by reference and resets their values.

Reviewed by: cem
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513