Use kvzalloc which attempts to allocate a physically continuous buffer and
fallbacks to virtually continuous on failure instead of open coding it in
the driver.

The is_vmalloc_addr function is used to determine whether the buffer is
physically continuous or not (which determines direct vs indirect MR
registration mode).
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Reviewed-by: Firas JahJah <firasj@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>