mds: be more explicit about path_traverse completion

Coverity turned up

CID 717085 (#1 of 1): Resource leak (RESOURCE_LEAK)
10. leaked_storage: Variable "c" going out of scope leaks the storage it points to.

from _find_ino_dir(), but for all r > 0 cases fin is consumed.  Make this
a bit more explicit by using the helper in the mdr-only case, and by
asserting it is zero in the return 2 path.  Hopefully coverity will notice.

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