os/bluestore: fix onode vs extent key suffix

os/bluestore: fix onode vs extent key suffix

I set the extent key suffix as 'x', thinking that was
not a valid hex character and would let us quickly
identify extent keys (vs onode keys, which end with an
encoded u64). But that doesn't work: the keys encode
integer values in raw form--not in hex
(pretty_binary_string just prints it out that way in
the debug log).

Fix by appending 'o' to the onode keys, so that we will
always have a trailing 'o' or 'x' and can use the last
char reliably to determine the type.

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