pstore: fs superblock limits

Leaving granularity at 1ns because it is dependent on the specific
attached backing pstore module. ramoops has microsecond resolution.

Fix the readback of ramoops fractional timestamp microseconds,
which has incorrectly been reporting the value as nanoseconds.

Fixes: 3f8f80f0 ("pstore/ram: Read and write to the 'compressed' flag of pstore").

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Cc: anton@enomsg.org
Cc: ccross@android.com
Cc: keescook@chromium.org
Cc: tony.luck@intel.com