IB/ipath: Fix a race where s_last is updated without lock held

There is a small window where a send work queue entry could be
overwritten by ib_post_send() because s_last is updated before the
entry is read.

This patch closes the window by acquiring the lock and updating
the last send work queue entry index after reading the wr_id.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>