xen: arm: correct use of find_next_bit

xen: arm: correct use of find_next_bit

find_next_bit takes a "const unsigned long *" but forcing a cast of an
"uint32_t *" throws away the alignment constraints and ends up causing an
alignment fault on arm64 if the input happened to be 4 but not 8 byte aligned.

Instead of casting use a temporary variable of the right type.

I've had a look around for similar constructs and the only thing I found was
maintenance_interrupt which cases a uint64_t down to an unsigned long, which
although perhaps not best advised is safe I think.

This was observed with the AArch64 Linaro toolchain 2013.12 but I think that
is just coincidental due to subtle changes to the stack layout etc.

Reported-by: Fu Wei <fu.wei@linaro.org>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>