rpc: remove some BUG()s

It would be kinder to WARN() and recover in several spots here instead
of BUG()ing.

Also, it looks like the read_u32_from_xdr_buf() call could actually
fail, though it might require a broken (or malicious) client, so convert
that to just an error return.

Reported-by: Weston Andros Adamson <dros@monkey.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>