9p: local: Properly set errp in fstatfs error path

9p: local: Properly set errp in fstatfs error path

In the review of

    9p: Avoid warning if FS_IOC_GETVERSION is not defined

Grep Kurz noted this error path was failing to set errp.
Fix that.

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
[added local: to commit title, Greg Kurz]
Signed-off-by: Greg Kurz <groug@kaod.org>