Pin SegmentMap block when they might need to be dirtied.

Pin SegmentMap block when they might need to be dirtied.

This is more in-keeping with other practices and not that SegmentMap
blocks are handles carefully by the cleaner and checkpoint, it is
safe to do this.
They stay pinned until they are no-longer referenced.  This may keep
some Credits unavailable but that is not a big cost.

When we pin these we don't hold or need a phase lock, so don't
require it.

Now that we alway pin segments when first used (free_get) we don't
need to prealloc in lafs_seg_move.

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