arm64: Make atomic64_t() return "long", not "long long"

arm64 sets CONFIG_64BIT=y and hence uses the "long counter" atomic64_t
definition from include/linux/types.h.  Make atomic64_read() return "long",
not "long long".

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>