IB/mlx5: Extract page fault code

To make page fault handling code more flexible
split pagefault_single_data_segment() function.
Keep MR resolution in pagefault_single_data_segment() and
move actual updates into pagefault_single_mr().

Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>