bcachefs: Set the last mount time using the realtime clock

bcachefs: Set the last mount time using the realtime clock

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>