linux: fsync on a directory should return 0, not EINVAL

linux: fsync on a directory should return 0, not EINVAL

Directory writes are synchronous, so this is fine. There's a
mostly-convenient function in fs/libfs.c that returns 0 that we can use
to do what we want ("mostly" because it was renamed in 2.6.35).

FIXES 130425

Reviewed-on: http://gerrit.openafs.org/6491
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 267934d0e6910c8d8166a6e78f93c1bab40857b8)

Change-Id: Iaeb8a699673b6144c186b470f6d877fb54f1e319
Reviewed-on: http://gerrit.openafs.org/6493
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>