ceph: fix a buffer leak in __ceph_setxattr()

In the Linux kernel, the following vulnerability has been resolved:

ceph: fix a buffer leak in __ceph_setxattr()

The old_blob in __ceph_setxattr() can store

ci->i_xattrs.prealloc_blob value during the retry.

However, it is never called the ceph_buffer_put()

for the old_blob object. This patch fixes the issue of

the buffer leak.