crush, mon: unlink vs remove

crush, mon: unlink vs remove

Make an 'unlink' mode of remove that will remove a link to a bucket but
not remove the bucket itself.  This refactors remove_item[_under] and moves
some of the checks into common helpers where they are not duplicated.  Fix
callers to pass the extra arg.

Signed-off-by: Sage Weil <sage@inktank.com>