nspawn: cleanup use of ERRNO_IS_NOT_SUPPORTED()

nspawn: cleanup use of ERRNO_IS_NOT_SUPPORTED()

Given that ERRNO_IS_NOT_SUPPORTED() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_NOT_SUPPORTED() is the
value returned by remount_idmap() which is not expected to return
any positive values, but let's be consistent anyway and move the
ERRNO_IS_NOT_SUPPORTED() invocation to the branch where
the return value is known to be negative.