pNFS: Ensure we layoutcommit before revalidating attributes

If we need to update the cached attributes, then we'd better make
sure that we also layoutcommit first. Otherwise, the server may have stale
attributes.

Prior to this patch, the revalidation code tried to "fix" this problem by
simply disabling attributes that would be affected by the layoutcommit.
That approach breaks nfs_writeback_check_extend(), leading to a file size
corruption.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>