Fix regression in bsdinstall post r356740 - partedit errno(2) 21 EISDIR

This resulted in the partitioning step failing if either of the
"Auto (UFS)" or "Manual" options were selected.

Reason: partedit was attempting to open a directory (TMPDIR) read/write,
which resulted in errno(2) 21 - EISDIR - Is a directory.

Reported by: Clay Daniels <clay.daniels.jr@gmail.com>
Reviewed by: Ryan Moeller <ryan@freqlabs.com>
Approved by: emaste, bcran
Differential Revision: https://reviews.freebsd.org/D23232