ceph-filestore-dump.cc: pass OSDSuperblock by reference

CID 1100693 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter superblock of type OSDSuperblock
 (size 240 bytes) by value.

CID 1100694 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter sb of type OSDSuperblock (size 240 bytes)
 by value.

CID 1100695 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter sb of type OSDSuperblock (size 240 bytes)
 by value.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>