osd: add config for osd_max_attr_name_len = 100

Set a limit on the length of an attr name.  The fs can only take 128
bytes, but we were not imposing any limit.

Add a test.

Reported-by: Haomai Wang <haomaiwang@gmail.com>
Signed-off-by: Sage Weil <sage@inktank.com>