Fix an uninitialized data access

zfs_acl_node_alloc allocates an uninitialized data buffer, but upstack
zfs_acl_chmod only partially initializes it.  KMSAN reported that this
memory remained uninitialized at the point when it was read by
lzjb_compress, which suggests a possible kernel memory disclosure bug.

The full KMSAN warning is:
```
panic: MSan: Uninitialized malloc memory from zfs_acl_chmod+0x2cd
cpuid = 6
time = 1725502132
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x99/frame 0xfffffe00b4c9c540
vpanic() at vpanic+0x56e/frame 0xfffffe00b4c9c6d0
panic() at panic+0x1dd/frame 0xfffffe00b4c9c7e0
__msan_warning() at __msan_warning+0x244/frame 0xfffffe00b4c9c930
lzjb_compress() at lzjb_compress+0x9f6/frame 0xfffffe00b4c9ca70
zio_compress_data() at zio_compress_data+0x388/frame 0xfffffe00b4c9cb40
zio_write_compress() at zio_write_compress+0x12bd/frame 0xfffffe00b4c9cca0
zio_execute() at zio_execute+0x4e0/frame 0xfffffe00b4c9cd30
taskqueue_run_locked() at taskqueue_run_locked+0x607/frame 0xfffffe00b4c9ce30
taskqueue_thread_loop() at taskqueue_thread_loop+0x29e/frame 0xfffffe00b4c9cea0
fork_exit() at fork_exit+0x1ee/frame 0xfffffe00b4c9cf30
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe00b4c9cf30
--- trap 0x5a5a5a5a, rip = 0x5a5a5a5a5a5a5a5a, rsp = 0x5a5a5a5a5a5a5a5a, rbp = 0x5a5a5a5a5a5a5a5a ---
KDB: enter: panic
```

Signed-off-by:	Alan Somers <asomers@gmail.com>
Sponsored by:	Axcient

module/os/freebsd/zfs/zfs_acl.c

@@ -473,7 +473,7 @@ zfs_acl_node_alloc(size_t bytes)
 
 	aclnode = kmem_zalloc(sizeof (zfs_acl_node_t), KM_SLEEP);
 	if (bytes) {
-		aclnode->z_acldata = kmem_alloc(bytes, KM_SLEEP);
+		aclnode->z_acldata = kmem_zalloc(bytes, KM_SLEEP);
 		aclnode->z_allocdata = aclnode->z_acldata;
 		aclnode->z_allocsize = bytes;
 		aclnode->z_size = bytes;