MDS: update the mlogger of mds in function check_memory_usage

MDS: update the mlogger of mds in function check_memory_usage

If we don't update the mlogger here, we will encounter following bug:
suppose we first create 10000 inode in mdcache, then we trimmed it to 1000
but the "ceph daemon mds.* perf dump" will not show the correct inode number, it still shows 10000.
because for now we only update the mlogger at MDSRank::_dispatch.
check_memory_usage is called right after "mdcache trimming" at every mds tick interval
so this change will keep "ceph daemon mds.* perf dump" got the right number for "mds_mem".

Signed-off-by: dongdong tao <tdd21151186@gmail.com>