arm64: fix return value type of memcpy_mcs()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6AQE8


CVE: NA

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The return value type of function memcpy_mcs() should be unsigned long.
Currently, using this function will force type conversion, but the error
of the return value type must be corrected.

Fixes: ee7b5d75 ("arm64: introduce copy_mc_to_kernel() implementation")
Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>