zfs_log: add flex array fields to log record structs

ZIL log record structs (lr_XX_t) are frequently allocated with extra
space after the struct to carry variable-sized "payload" items.

Linux 6.10+ compiled with CONFIG_FORTIFY_SOURCE has been doing runtime
bounds checking on memcpy() calls. Because these types had no indicator
that they might use more space than their simple definition,
__fortify_memcpy_chk will frequently complain about overruns eg:

    memcpy: detected field-spanning write (size 7) of single field
        "lr + 1" at zfs_log.c:425 (size 0)
    memcpy: detected field-spanning write (size 9) of single field
        "(char *)(lr + 1)" at zfs_log.c:593 (size 0)
    memcpy: detected field-spanning write (size 4) of single field
        "(char *)(lr + 1) + snamesize" at zfs_log.c:594 (size 0)
    memcpy: detected field-spanning write (size 7) of single field
        "lr + 1" at zfs_log.c:425 (size 0)
    memcpy: detected field-spanning write (size 9) of single field
        "(char *)(lr + 1)" at zfs_log.c:593 (size 0)
    memcpy: detected field-spanning write (size 4) of single field
        "(char *)(lr + 1) + snamesize" at zfs_log.c:594 (size 0)
    memcpy: detected field-spanning write (size 7) of single field
        "lr + 1" at zfs_log.c:425 (size 0)
    memcpy: detected field-spanning write (size 9) of single field
        "(char *)(lr + 1)" at zfs_log.c:593 (size 0)
    memcpy: detected field-spanning write (size 4) of single field
        "(char *)(lr + 1) + snamesize" at zfs_log.c:594 (size 0)

To fix this, this commit adds flex array fields to all lr_XX_t structs
that require them, and then uses those fields to access that
end-of-struct area rather than more complicated casts and pointer
addition.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16501
Closes #16539

cmd/zdb/zdb_il.c

@@ -64,7 +64,8 @@ static void
 zil_prt_rec_create(zilog_t *zilog, int txtype, const void *arg)
 {
 	(void) zilog;
-	const lr_create_t *lr = arg;
+	const lr_create_t *lrc = arg;
+	const _lr_create_t *lr = &lrc->lr_create;
 	time_t crtime = lr->lr_crtime[0];
 	char *name, *link;
 	lr_attr_t *lrattr;
@@ -121,7 +122,8 @@ static void
 zil_prt_rec_rename(zilog_t *zilog, int txtype, const void *arg)
 {
 	(void) zilog, (void) txtype;
-	const lr_rename_t *lr = arg;
+	const lr_rename_t *lrr = arg;
+	const _lr_rename_t *lr = &lrr->lr_rename;
 	char *snm = (char *)(lr + 1);
 	char *tnm = snm + strlen(snm) + 1;