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
Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 49ff63b1750789070a8c6fef830c9526ae0f6d9f)