RDMA/mlx5: Fix incorrect MKEY masking

RDMA/mlx5: Fix incorrect MKEY masking

mkey_mask is __be64 type, while MLX5_MKEY_MASK_PAGE_SIZE is declared as
unsigned long long. This causes to the static checkers errors:

drivers/infiniband/hw/mlx5/umr.c:663:49: warning: invalid assignment: &=
drivers/infiniband/hw/mlx5/umr.c:663:49:    left side has type restricted __be64
drivers/infiniband/hw/mlx5/umr.c:663:49:    right side has type int

Fixes: e73242aa14d2 ("RDMA/mlx5: Optimize DMABUF mkey page size")
Link: https://patch.msgid.link/e354d70b98dfa5ecf4c236a36cd36b64add9d9de.1753003467.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>