rados_sync.cc: fix xattr_diff() for the only_in_b checks

In the checks to build only_in_b up the wrong const_iterator x is
build up. it should compare rhs->xattrs with xattrs entries and
not twice rhs->xattrs.

Fix for:

CID 716957 (#1 of 1): Invalid iterator comparison (MISMATCHED_ITERATOR)
 mismatched_comparison: Comparing x from rhs->xattrs to this->xattrs.end()
 from this->xattrs.

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