XSK redirecting XDP programs require linearity, hence applies
restrictions on the MTU. For PAGE_SIZE=4K, MTU shouldn't exceed 3498.

Features that contradict with XDP such HW-LRO and HW-GRO are enforced
by the driver in advance, during XSK params validation, except for MTU,
which was not enforced before this patch.

This has been spotted during test scenario described below:
Attaching xdpsock program (PAGE_SIZE=4K), with MTU < 3498, detaching
XDP program, changing the MTU to arbitrary value in the range
[3499, 3754], attaching XDP program again, which ended up with failure
since MTU is > 3498.

This commit lowers the XSK MTU limitation to be aligned with XDP MTU
limitation, since XSK socket is meaningless without XDP program.
Signed-off-by: Adham Faris <afaris@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>