tools: rados put /dev/null should write() and not create()

In the rados.cc special case to handle put an empty objects, use
write_full() instead of create().

A special case was introduced 6843a0b81f10125842c90bc63eccc4fd873b58f2
to create() an object if the rados put file is empty. Prior to this fix
an attempt to rados put an empty file was a noop. The problem with this
fix is that it is not idempotent. rados put an empty file twice would
fail the second time and rados put a file with one byte would succeed as
expected.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2626 from dachary/wip-rados-empty-file

tools: rados put /dev/null should write() and not create()

Reviewed-by: Sage Weil <sage@redhat.com>