um: Cleanup syscall_handler_t cast in syscalls_32.h

stable inclusion
from stable-v5.10.148
commit 6d4deaba063d3dc6c41ccd86f076fefdd8b34f81
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6D0WL

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d4deaba063d3dc6c41ccd86f076fefdd8b34f81



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[ Upstream commit 61670b4d ]

Like in f4f03f29
"um: Cleanup syscall_handler_t definition/cast, fix warning",
remove the cast to to fix the compiler warning.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>