drivers/fpga: use standard array-copy function

dfl.c utilizes memdup_user() and array_size() to copy a userspace array.
array_size() will likely never trigger thanks to the preceding check.
Nevertheless, in the theoretical event that it would, it would return
SIZE_MAX to memdup_user(), resulting in an attempt to allocate huge
amounts of memory.

string.h from the core-api now provides memdup_array_user() which also
performs an overflow check and returns an error-pointer with -EOVERFLOW
to the caller.
As an additional advantage it standardizes how userspace-arrays are
being copied and, thus, makes it more obvious to readers that an array
is being copied.

Replace memdup_user() with memdup_array_user().

Suggested-by: Dave Airlie <[email protected]>
Signed-off-by: Philipp Stanner <[email protected]>
Acked-by: Xu Yilun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Xu Yilun <[email protected]>

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