get_xattr() can return more than 4KB

Instead of failing if the attribute to be returned is larger than 4KB,
double the buffer size each time librados.rados_getxattr returns
-errno.ERANGE and try again.

http://tracker.ceph.com/issues/4907 fixes #4907

Signed-off-by: Loic Dachary <loic@dachary.org>