IB/core: Fix user mode post wr corruption

Commit e622f2f4 ("IB: split struct ib_send_wr")
introduced a regression for HCAs whose user mode post
sends go through ib_uverbs_post_send().

The code didn't account for the fact that the first sge is
offset by an operation dependent length.  The allocation did,
but the pointer to the destination sge list is computed without
that knowledge.  The sge list copy_from_user() then corrupts
fields in the work request

Store the operation dependent length in a local variable and
compute the sge list copy_from_user() destination using that length.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>