Add support for mmap() method to mthca, so that userspace can get access to
doorbell registers.  This allows userspace to get direct access to the HCA for
data path operations.

Each userspace context gets its own copy of the doorbell registers and is only
allowed to use resources that the kernel has given it access to.  In other
words, this is safe.
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>