net/mlx5e: Allow XSK frames smaller than a page

Relax the requirements to the XSK frame size to allow it to be smaller
than a page and even not a power of two. The current implementation can
work in this mode, both with Striding RQ and without it.

The code that checks `mtu + headroom <= XSK frame size` is modified
accordingly. Any frame size between 2048 and PAGE_SIZE is accepted.

Functions that worked with pages only now work with XSK frames, even if
their size is different from PAGE_SIZE.

With XSK queues, regardless of the frame size, Striding RQ uses the
stride size of PAGE_SIZE, and UMR MTTs are posted using starting
addresses of frames, but PAGE_SIZE as page size. MTU guarantees that no
packet data will overlap with other frames. UMR MTT size is made equal
to the stride size of the RQ, because UMEM frames may come in random
order, and we need to handle them one by one. PAGE_SIZE is just a power
of two that is bigger than any allowed XSK frame size, and also it
doesn't require making additional changes to the code.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>