ceph: improve error handling in ceph_mdsmap_decode

This patch makes the following improvements to the error handling
in the ceph_mdsmap_decode function:

- Add a NULL check for return value from kcalloc
- Make use of the variable err

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Sage Weil <sage@inktank.com>