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