RDMA/efa: Use offset_in_page() function

Use offset_in_page() instead of open-coding it.

Link: https://lore.kernel.org/r/20240624160918.27060-2-mrgolin@amazon.com


Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Reviewed-by: Firas Jahjah <firasj@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Michael Margolin <mrgolin@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>