asm-generic: fix ffs -Wshadow warning

mainline inclusion
from mainline-v5.11
commit 6f6573a4
category: bugfix
bugzilla: 176150 https://gitee.com/openeuler/kernel/issues/I4DDEL

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6f6573a4044adefbd07f1bd951a2041150e888d7



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gcc -Wshadow warns about the ffs() definition that has the
same name as the global ffs() built-in:

include/asm-generic/bitops/builtin-ffs.h:13:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]

This is annoying because 'make W=2' warns every time this
header gets included.

Change it to use a #define instead, making callers directly
reference the builtin.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Zhang Jianhua <chris.zjh@huawei.com>
Reviewed-by: He Ying <heying24@huawei.com>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>