Add V_MNTREF flag to the vn_start_write(9) and
vn_start_secondary_write(9) functions. The flag indicates that the
caller already owns a reference on the mount point, and the functions
can consume it. The reference is released by vn_finished_write(9) and
vn_finished_secondary_write(9) in due course.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks