libafs: Do not osi_FlushPages for dirs

libafs: Do not osi_FlushPages for dirs

Directory contents are never mapped or stored in pages, so dealing
with page invalidation on directories is just overhead. So make
osi_FlushPages a no-op when we're given a directory, which can avoid a
lot of locks and other processing (particularly when we are called in
afs_getattr in BOZONLOCK_ENV).

Change-Id: I56fbd08c74d01a5fcb45a57e08bb31afcec9331d
Reviewed-on: http://gerrit.openafs.org/4259
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>