mountd: fix exporting of "/" with sec= setting.

Commit 91bb95f2689e84856ecdf6fac365489d36709cf9
   4set_root: force "fsid=0" for all exports of '/'

set NFSEXP_FSID for the export of "/" if nothing else had any fsid set,
however it didn't also set the flag for all security flavours.  So the
kernel complains that the flags on the security flavours don't match and
it rejects the export.

So call fix_pseudoflavor_flags() in write_secinfo() to make sure that
any fiddling that has been done to e_flags gets copied to e_secinfo.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>