Add --enable-asan and --enable-ubsan switches

`configure` now accepts `--enable-asan` and `--enable-ubsan` switches
which results in passing `-fsanitize=address`
and `-fsanitize=undefined`, respectively, to the compiler. Those
flags are enabled in GitHub workflows for ZTS and zloop. Errors
reported by both instrumentations are corrected, except for:

- Memory leak reporting is (temporarily) suppressed. The cost of
  fixing them is relatively high compared to the gains.

- Checksum computing functions in `module/zcommon/zfs_fletcher*`
  have UBSan errors suppressed. It is completely impractical
  to enforce 64-byte payload alignment there due to performance
  impact.

- There's no ASan heap poisoning in `module/zstd/lib/zstd.c`. A custom
  memory allocator is used there rendering that measure
  unfeasible.

- Memory leaks detection has to be suppressed for `cmd/zvol_id`.
  `zvol_id` is run by udev with the help of `ptrace(2)`. Tracing is
  incompatible with memory leaks detection.

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12928

module/zfs/vdev.c

@@ -4531,8 +4531,8 @@ vdev_stat_update(zio_t *zio, uint64_t psize)
 	vdev_t *vd = zio->io_vd ? zio->io_vd : rvd;
 	vdev_t *pvd;
 	uint64_t txg = zio->io_txg;
-	vdev_stat_t *vs = &vd->vdev_stat;
-	vdev_stat_ex_t *vsx = &vd->vdev_stat_ex;
+	vdev_stat_t *vs = vd ? &vd->vdev_stat : NULL;
+	vdev_stat_ex_t *vsx = vd ? &vd->vdev_stat_ex : NULL;
 	zio_type_t type = zio->io_type;
 	int flags = zio->io_flags;