Add a mthca_write_db_rec() to wrap writing doorbell records.  On
64-bit archs, this is just a 64-bit write, while on 32-bit archs it
splits the write into two 32-bit writes with a memory barrier to make
sure the two halves of the record are written in the correct order.
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>