atime: make sure block is reserved before updating.

atime: make sure block is reserved before updating.

We need to 'reserve' space for the atime file blocks before updating
them.  This could of course fail.  If it does we could lose the
update, but as long as that is very rare it shouldn't be a problem.

When we fix things so the block hardly ever gets written, we should
also fix it to reserve the block when we first take the reference.
Then also refresh the reservation after writing.

Also fix up some comments and stuff.

Signed-off-by: NeilBrown <neilb@suse.de>