rgw: fix memory fragmentation problem reading data from client.

rgw: fix memory fragmentation problem reading data from client.

mg_read returns 0 on EOF.  In some versions of civetweb, mg_read can
return "short" reads.  The logic in the rest of ceph depends on a read
always filling its buffer when possible.  So loop here and fill the
buffer.  Looping to fill the buffer means we must also track when
mg_read returns EOF, because after that further calls to mg_read will return -1.

Fixes: https://tracker.ceph.com/issues/23207
Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit d9a150b1f1f9ec3952555186722c9b13d7448e2d)

Conflicts:
    src/rgw/rgw_civetweb.cc:
        - master RGWCivetWeb::RGWCivetWeb() becomes RGWMongoose::RGWMongoose() in jewel,
          with slightly different initializer list; adapt mg_read() call for jewel
        - master RGWCivetWeb::read_data() becomes RGWMongoose::read_data() in jewel,
          and inside this function we return -EIO in jewel, instead of throwing an exception