install: re-check failed mkdir for EEXIST

Since the code stats and mkdirs in 2 separate steps, it is possible that
the directory will be created in the meantime by something else (e.g.
concurrent install).[1]

While here alter the code to properly report stat failure, previously it
would always claim it was mkdir which failed.

Noted by: royger [1]
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272026 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f