xsk: Fix number of pinned pages/umem size discrepancy

For AF_XDP sockets, there was a discrepancy between the number of of
pinned pages and the size of the umem region.

The size of the umem region is used to validate the AF_XDP descriptor
addresses. The logic that pinned the pages covered by the region only
took whole pages into consideration, creating a mismatch between the
size and pinned pages. A user could then pass AF_XDP addresses outside
the range of pinned pages, but still within the size of the region,
crashing the kernel.

This change correctly calculates the number of pages to be
pinned. Further, the size check for the aligned mode is
simplified. Now the code simply checks if the size is divisible by the
chunk size.

Fixes: bbff2f321a86 ("xsk: new descriptor addressing scheme")
Reported-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200910075609.7904-1-bjorn.topel@gmail.com

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