RDMA/cma: Verify private data length

private_data_len is defined as a u8.  If the user specifies a large
private_data size (> 220 bytes), we will calculate a total length that
exceeds 255, resulting in private_data_len wrapping back to 0.  This
can lead to overwriting random kernel memory.  Avoid this by verifying
that the resulting size fits into a u8.

Reported-by: B. Thery <benjamin.thery@bull.net>
Addresses: <http://bugs.openfabrics.org/bugzilla/show_bug.cgi?id=2335

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Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>