mds: use const ref instead of large pass-by-value

mds: use const ref instead of large pass-by-value

CID 717033 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
Passing parameter inst of type entity_inst_t (size 152 bytes) by value.

CID 717034 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
Passing parameter inst of type entity_inst_t (size 152 bytes) by value.

CID 717032 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
Passing parameter i of type entity_inst_t (size 152 bytes) by value.

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