uapi: simplify __ARCH_FLOCK{,64}_PAD a little

uapi: simplify __ARCH_FLOCK{,64}_PAD a little

Don't bother to define the symbols empty, just don't use them.
That makes the intent a little more clear.

Remove the unused HAVE_ARCH_STRUCT_FLOCK64 define and merge the
32-bit mips struct flock into the generic one.

Add a new __ARCH_FLOCK_EXTRA_SYSID macro following the style of
__ARCH_FLOCK_PAD to avoid having a separate definition just for
one architecture.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-2-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>