zap_impl: use flex array field for mzap_phys_t.mz_chunks

mz_phys_t is always a full-block allocation, with mz_chunks[] as an
array over the rest of the block past the header.

Recent Linux compiled with CONFIG_UBSAN will complain about this:

    UBSAN: array-index-out-of-bounds in module/zfs/zap.c:1236:28
    index 2 is out of range for type 'mzap_ent_phys_t [1]'

The fix is straightforward; simply convert this field to a flex member.

Sponsored-by: TrueNAS
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18550

include/sys/zap_impl.h

@@ -62,8 +62,9 @@ typedef struct mzap_phys {
 	uint64_t mz_salt;
 	uint64_t mz_normflags;
 	uint64_t mz_pad[5];
-	mzap_ent_phys_t mz_chunk[1];
+
 	/* actually variable size depending on block size */
+	mzap_ent_phys_t mz_chunk[];
 } mzap_phys_t;
 
 typedef struct mzap_ent {