Add plfs_flush and plfs_invalidate_cache to manage internal cache

plfs_flush will flush all the buffers related to a pathname, so
that all the changes made by this process are written to disks.

plfs_invalidate_cache will drop all the caches related to a
pathname so that later operations will get the latest info
about this file from disks.

Together they can provide a flush to invalidate_cache consistency
for small file mode.