RDMA/rdmavt: Add driver mmap callback

RDMA/rdmavt: Add driver mmap callback

Add a reserved range and a driver callback to allow the driver to
have custom mmaps.

Generated mmap offsets are cookies and are not related to the size of
the mmap.  Advance the mmap offset by the minimum, PAGE_SIZE, rather
than the size of the mmap.

Signed-off-by: Dean Luick <dean.luick@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Link: https://patch.msgid.link/177308909972.1279894.15543003811821875042.stgit@awdrv-04.cornelisnetworks.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>