locking: Move MCS struct definition to public header

locking: Move MCS struct definition to public header

Move the definition of the struct mcs_spinlock from the private
mcs_spinlock.h header in kernel/locking to the mcs_spinlock.h
asm-generic header, since we will need to reference it from the
qspinlock.h header in subsequent commits.

Reviewed-by: Barret Rhoden <brho@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>