IB/ehca: Use remap_4k_pfn() to map firmware contexts to user space

Use Paul's new remap_4k_pfn() function to map our 4K firmware contexts
into user space on 64K-page machines without exposing neighboring
firmware contexts. Return the context's offset within a 64K page to
user space so it can determine the proper virtual address.

For details about remap_4k_pfn(), see commit 721151d0 or
http://patchwork.ozlabs.org/linuxppc/patch?id=10281

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>