rgw: add RGWFormatter_Plain allocation to sidestep cranky strlen()

Valgrind complains about an invalid read when we don't pad the allocation,
and because it is inlined we can't whitelist it for valgrind.  Workaround
the warning by just padding our allocations a bit.

Fixes: #5346
Signed-off-by: Sage Weil <sage@inktank.com>