ceph: fix listxattr handling for vxattrs

Only include vxattrs in the result if they are not hidden and exist
(as determined by the exists_cb callback).

Note that the buffer size we return when 0 is passed in always includes
vxattrs that *might* exist, forming an upper bound.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>