dlm: make find_rsb() fail gracefully when namelen is too large

We *can* get there from receive_request() and dlm_recover_master_copy()
with namelen too large if incoming request is invalid; BUG() from
DLM_ASSERT() in allocate_rsb() is a bit excessive reaction to that
and in case of dlm_recover_master_copy() we would actually oops before
that while calculating hash of up to 64Kb worth of data - with data
actually being 64 _bytes_ in kmalloc()'ed struct.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Teigland <teigland@redhat.com>