osd: add config for osd_max_object_name_len = 2048 (was hard-coded at 4096)

Previously we had a hard coded limit of 4096.  Objects > 3k crash the OSD 
when running on ext4, although they probably work on xfs.  But rgw only 
generates objects a bit over 1024 bytes (maybe 1200 tops?), so let set a more
reasonable limit here.  2048 is a nice round number and should be safe.

Add a test.

Fixes: #8174 Signed-off-by: Sage Weil <sage@redhat.com>