mds: fix subtree_map simplify, again
mds: fix subtree_map simplify, again

Erasing from the vector<> seems to be clobbering the iterator, even when it
preceeds the deletion point.  Use integer index instead. Yuck!

Signed-off-by: Sage Weil <c5b6ff1fd4f0bde10db63d7a3e07166cf6c4229f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph