Fix early kernel dump via dumpdev env

Setting the dumpdev via env e.g. loader.conf provides the ability to
configure the kernel dump device during early boot. When using this
g_io_getattr was returning EPERM due to cp->acr == 0.

Fix this by calling g_access to ensure we're a read consumer prior
to calling g_dev_setdumpdev.

MFC after: 2 weeks
Sponsored by: Multiplay