pipe: Avoid unnecessary priv_check() calls in pipespace_new()

Running out of pipe map KVA is a rare case, so reorder checks
accordingly, presuming that calling priv_check() is more expensive than
the calculation. In particular, priv_check() might not be cheap to
evaluate if MAC hooks are installed.

Reviewed by: olce, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55378