fs: ensure that ->getattr() honors AT_STATX_CACHED

fs: ensure that ->getattr() honors AT_STATX_CACHED

For filesystems that provide a private ->getattr() implementation, some of
them need to do IO to satisfy the request. If we need to block off
->getattr() and AT_STATX_CACHED is set, then return -EAGAIN and have the
caller retry.

Signed-off-by: Jens Axboe <axboe@kernel.dk>