Preallocate pipe buffers on pipe creation.

Return ENOMEM if one of the buffer cannot be created even with the
minimal size. This should avoid subsequent spurious ENOMEM errors
from write(2) when buffer cannot be allocated on the fly, after we
reported that the pipe was create succesfully.

Reported by: Keno Fischer <keno@juliacomputing.com>
Reviewed by: markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23993