This way the last mount time is actually meaningful instead of just being
various times from 1970 (which happens with the monotonic clock).

Also, roundup_pow_of_two() is undefined when passed in 0, so check before
calling it.
Signed-off-by: Tim Schlueter <schlueter.tim@linux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>