treewide: uapi: Replace zero-length arrays with flexible-array members

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

This code was transformed with the help of Coccinelle:
(linux-5.19-rc2$ spatch --jobs $(getconf _NPROCESSORS_ONLN) --sp-file script.cocci --include-headers --dir . > output.patch)

@@
identifier S, member, array;
type T1, T2;
@@

struct S {
  ...
  T1 member;
  T2 array[
- 0
  ];
};

-fstrict-flex-arrays=3 is coming and we need to land these changes
to prevent issues like these in the short future:

../fs/minix/dir.c:337:3: warning: 'strcpy' will always overflow; destination buffer has size 0,
but the source string has length 2 (including NUL byte) [-Wfortify-source]
		strcpy(de3->name, ".");
		^

Since these are all [0] to [] changes, the risk to UAPI is nearly zero. If
this breaks anything, we can use a union with a new member name.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.16/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/78
Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/62b675ec.wKX6AOZ6cbE71vtF%25lkp@intel.com/
Acked-by: Dan Williams <dan.j.williams@intel.com> # For ndctl.h
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

tools/include/uapi/linux/kvm.h

@@ -539,7 +539,7 @@ struct kvm_coalesced_mmio {
 
 struct kvm_coalesced_mmio_ring {
 	__u32 first, last;
-	struct kvm_coalesced_mmio coalesced_mmio[0];
+	struct kvm_coalesced_mmio coalesced_mmio[];
 };
 
 #define KVM_COALESCED_MMIO_MAX \
@@ -618,7 +618,7 @@ struct kvm_clear_dirty_log {
 /* for KVM_SET_SIGNAL_MASK */
 struct kvm_signal_mask {
 	__u32 len;
-	__u8  sigset[0];
+	__u8  sigset[];
 };
 
 /* for KVM_TPR_ACCESS_REPORTING */
@@ -1216,7 +1216,7 @@ struct kvm_irq_routing_entry {
 struct kvm_irq_routing {
 	__u32 nr;
 	__u32 flags;
-	struct kvm_irq_routing_entry entries[0];
+	struct kvm_irq_routing_entry entries[];
 };
 
 #endif
@@ -1335,7 +1335,7 @@ struct kvm_dirty_tlb {
 
 struct kvm_reg_list {
 	__u64 n; /* number of regs */
-	__u64 reg[0];
+	__u64 reg[];
 };
 
 struct kvm_one_reg {