pipe: keep uio_iovcnt consistent

In pipe_build_write_buffer we increment uio_iov but did not update
uio_iovcnt. This would not cause an OOB read (thanks to to uio_resid)
but is inconsistent and could be an issue if other code changes are made
in the future.

Reported by: Synacktiv
Reviewed by: jhb, markj, brooks
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45999

(cherry picked from commit d8ff42e816848a0d4a427755b46b8560cb86ebc8)
(cherry picked from commit cbbc8d67301c650dd683566d44ff554993556224)
(cherry picked from commit 0fb9da95f7cb1730fdf4308f9a12ff8a80c28c87)

Approved by: re (cperciva)