os/KeyValueStore: make get_max_object_name_length() sane

This is getting the NAME_MAX from the OS, but in reality the backend
KV store is the limiter.  And for leveldb, there is no real limit.
Return 4096 for now.

Signed-off-by: Sage Weil <sage@redhat.com>
os: make name/attr max methods unsigned

This fixes warnings when we use these in MIN/MAX macros against
other unsigned values.

Fixes: #8897
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2133 from ceph/wip-8897

os: fix build warnings with name/attr len checks (fixes 8889)

Reviewed-by: Greg Farnum <greg@inktank.com>